For runs it's mostly just for fun. I use Strava so it has sections they identify along your run that they rank you against your friends and strangers. I do end up running very similar routes most of the time.

I mostly find the GPS invaluable in hiking. It can be a great thing to have on the top of a mountain in the winter with low visibility. If you get up to a summit and the weather turns, you can have the watch send you back down the way you came. Nice and safe.