I have a Garmin and upload my ride data to Strava.
I use Strava because I like being able to see how my performance on climbs improves with increased fitness (and skill, in the case of mountain biking). I can create segments of my rides to see how I'm doing, and also see my progress on segments that other people have created.
Strava also enables you to see all the created segments in an area of the map, which helps me to find new and challenging climbs, and I also like to look at the routes other people are riding, which gives me ideas of other places to ride that I might not have considered before.
As you may have gathered, I spend a lot of time on Strava!