I disagree that an altimeter is important. While a hilly ride does take more energy than a flat ride, for a given number of miles, most of us are not really out to find out how many joules we spend on a ride. Comparing your time from one day to the next on a given route is interesting. Comparing your distance from one day to the next is also interesting. Watching your average speed increase as your conditioning improves is interesting and useful. You can do all of these with a cheap computer. For that matter, you can do them with a watch and some paper and pen, but the computer makes it easier.