When men learned to fly, pilots didn't need instruments such as compass, GPS or altimeter. Pilots hardly got off the ground, let alone far. And if they were (un)lucky to do so, a quick glance at the ground sufficed to estimate their altitude. The need for more sophisticated means arose when planes got better and better. When those flimsy experimental toys from the beginning had evolved into useful machines that could fly (long) distances for transporting passengers or dropping bombs on major cities, instead of just pointlessly bumping around on the local meadows.
After a century of remarkable progress in aviation, paragliding pilots have rediscovered that long forgotten pleasure of just locally bumping around on low performance toys. The fun of soaring the ridge near launch or just gliding down the valley, feeling the amazement that we can actually fly. A similar enthusiasm and joy that Orville and Wilbur must have had. There is no need for sophisticated equipment when flying a paraglider, considering its modest performance, but many pilots seem addicted to an overdose of instruments. Which is a bit of a sad thing, considering the natural clues that nature provides for navigation and thermalling. Flying without vario and GPS, I do not consider myself an addict. But I have to confess that, for various reasons, I carry an altimeter most of the time, even though I could probably do without it.
First of all, perhaps surprisingly, it serves as a weather station. Like most altimeters, it is pressure based. It does not give the real altitude as measured in metres, but an approximate altitude based on atmospheric pressure as given by the International Standard Atmosphere (ISA). The latter has been adopted and extended by the ICAO in order to facilitate flying rules. Using relative altitude (pressure) rather than actual altitude (distance) means that all aircraft use the same values for a specific altitude, independent of their geological location. This simple but effective system greatly mitigates collision risk. Up till now, it has remained superior to measuring the actual altitude, despite the technological progress since the early days of flying.
In the Alps you could easily do without an altimeter, since the surrounding mountains give an adequate impression of your altitude, but it helps in preventing any confusion when flying near the limits of VFR airspace or when you don't feel like memorising mountain heights. My altimeter is calibrated at 1013,2 hPa, which almost equals the average sea-level pressure of 1013,25 hPa from which flight level (FL) is calculated. Setting it to QFE (Field Elevation, zero altitude related to the atmospheric pressure on a location such as a launch or landing field), seems silly to me when going cross-country. Most pilots seem to prefer setting their altitude to QNH (Natural Height, zero altitude related to the atmospheric pressure at sea level), but this requires a constant recalibration with the actual altitude as the weather changes and thereby eliminates the advantage of having a weather station with me. And in order to recalibrate, you will need an indication of your current altitude. This could be given by a map, signpost, or building for example (railway stations usually have an altitude marker on the platform side), but these are not always around or precise enough. In the mountains, a slight deviation from your estimated position on the map could easily mean a deviation of tens of metres and possibly much more. Also remember that the actual altitude differs from the barometric altitude and that weather changes could shrink or extend altitude differences (by low and high pressure respectively).
An altimeter calibrated at FL0 helps me forecasting the weather, as well as keeping out of IFR and similar airspace. The lower limits for IFR flights start from FL115 up to FL195 in the French Alps, but these come down near airports and the extremities of the Alps. The ceiling for VFR flight is FL65 near Gourdon (Nice airport) and FL60 above Salève (Geneva airport). The area west of the Durance river near Sisteron has a much higher ceiling when the sailplane wave zones are active. And these are just a few examples. If you are using QNH or QFE settings here, then be aware that a change in the weather could get you into forbidden airspace if you fly on its limits. One hectopascal change in pressure roughly means eight metres change at lower altitudes. Pressure is likely to vary during the day, especially if there is a change in the weather. Rising pressure pushes flight levels up, while dropping pressure brings them down. The latter is more problematic than the former, since it shrinks our margin. Calibrating your altimeter at QNH just before the arrival of a depression for example, is not a good idea when flying near the limits of restricted airspace. Staying 150 to 200 m below the maximum altitude indicated by your altimeter, is likely to keep you out of trouble most of the time. Adjust this margin if you are unsure what the weather is doing and in order to compensate for the imprecision of your altimeter.
As mentioned earlier on, barometric altimeters are not only handy for estimating altitude, but also (as the name suggests) for forecasting the weather. Often being in the middle of nowhere during a vol bivouac, lacking communication means with the civilised world, I have come to rely on my own weather forecasts. Monitoring the altitude changes shown over time, but at the same altitude, is a great help for forecasting correctly. It may not be high-tech or very precise, but the service is free and adequate. Waking up with a steady and significant drop in altitude since going to sleep last night (values are logged automatically every half hour, up till seven days ago), and the sky having become overcast, my plans for the day are likely to concern searching for shelter rather than waiting for the clouds to disappear and fly.
Predicting the weather with an altimeter works a bit counter-intuitive in the beginning. A drop in altitude means a rise in pressure, while a rise means a drop. It takes some time getting used to this seemingly illogical peculiarity, but after a while you will be able to make a weather forecast without tiring your brain. And even if you do happen to confuse a rising altitude with rising pressure and thus nice weather, then the actual weather will soon correct this mistake with grey clouds and rain. Luckily, the reverse mistake is less depressive.
The third reason for having an altimeter with me, is that it helps me assessing the current ceiling, especially on a cloudless day (blue thermals). Losing a thermal near 1800 m altitude three times in a row, probably means I have reached the maximum altitude for the moment. Instead of searching in vain for more altitude gain, it is time to transit to the next thermal. Needing altitude while losing a thermal at 1500 m however, will have me re-centring in order to work my way up the last 300 m. The ceiling varies during the day and the terrain I am travelling through. Having an altimeter to precisely monitor the ceiling changes over time, will give me a better feel of the thermal development. Especially when coming back to a thermal spot that I passed earlier on. With a high cloud base for example, I find it harder to remember visual clues than the values shown by my altimeter. The start of thermal decline, telling me that certain roads will be closed soon, is often indicated by small altitudes differences that are hard to spot by sight alone if there is no vertical reference nearby.
Last and least, the altimeter helps me when natural clues for thermalling are confusing. Luckily this seldom happens, a few times a year. Mostly in weak lift during a (long) make-or-brake transition. With vertical references being far away, it takes a few turns for the result of the background check to rise above the noise. Gaining a little height by thermalling (no matter how small), could tip the balance here. Losing tens of metres in order to find out that the thermal is not really exploitable, tips it the wrong way. A quick look on my watch when entering the presumed 'thermal' and again after a few turns, increases my chances of making it. It helps in quickly determining whether I found a real thermal, a place with less sink, or just turbulence.
Unfortunately, my altimeter has the tendency to block now and then when subjected to sudden changes, i.e. when entering/exiting a thermal or turbulence. The sudden change in altitude seems to upset the altimeter, causing it to display the last know 'good' value. After (half) a minute or so, it gets over this shock and suddenly displays the actual altitude. Which could be much lower than expected if you did not pay attention to the natural clues surrounding you. Initially I cursed the manufacturer for this bug, but it has shown to be a blessing in disguise. It has forced me to improve my background checking skills. Now I am often able to note whether I am going up (or down) after a few turns in the middle of a wide valley, even though my altimeter keeps on indicating I am turning in zero lift. The trusted background check never gets upset and I have become to rely on my altimeter even less than before. I guess I was just being lazy. A fool with a tool is still a fool. Instruments often hide human imperfections, while practice breeds mastery for doing without them.
My altimeter is incorporated in a watch, making it small and light. It is on my wrist, instead of adding weight to the already heavy backpack. I have never worn a watch before I got this one and could certainly do without. I mean, what is the use of knowing the time with second precision when nature works with a much more general time scale? Failing to find a good and lightweight altimeter, I stumbled upon a watch with incorporated altimeter and compass that was more fit for my purposes than any of the stand alone altimeters. The digits are easy to read, operation is simple, and it has no bloatware or function fungus. The battery lasts a few months, its lifetime mainly depending on whether I use the built-in electronic compass or not. Sometimes the back of my hand accidentally activates the 'mode' button, switching the watch from altimeter to compass mode, which eats energy. This could be prevented by locking the buttons, but that requires pushing the 'light' button for two seconds, after which the display remains illuminated for another five seconds. Each lock-unlock cycle thus lasts fourteen seconds minimally and drains far more energy than needed. Probably less compared to the accidental compass energy drain, that automatically shuts-off after a minute, but just as unnecessary. And since (un)locking happens far more frequently than accidentally starting the compass, I stopped using it. The mode button is tiny, but can be operated during flight with my gloves on if I accidentally switch to another mode. The watch might not be perfect, but I like it and it is more than adequate for my purposes. Show me something that is perfect, and I will show you its defaults.