I like having the data (I'm a scientist, which has something to do with it too). With GPS I know where and how far I ran (I live near woods with a number of trails and don't always run the same route), how fast I was at different segments of a run, and so forth. It is really helpful when I want to run, bike, or walk a particular distance - I can just go and not have to plan route before. This makes things more interesting. Maybe it's just my short attention span.
From your other comments I can tell you don't see the need. That's fine. I use GPS tracking (and heart rate, sleep tracking, etc.) to improve my health activities. I have a large database with much of my health-related data (exercise, sleep, weight, calorie expenditure, heart rates, estimated VO2 Max...) in it for the past year (and counting). Is it necessary? No. It is useful and interesting to me? Yes. Is broader collection and analysis of these data going to be useful from a public health standpoint? Yes, within the next decade. Am I a data geek? Yes.