Running, for one. Not having to carry a phone is useful. Yes, there are hundreds of fitness trackers. Why not a multi-purpose tracker that also lets me reply to the wife?
Unfortunately as other people have pointed out, you do need to carry the phone. Which has already killed the Apple Watch for one runner I know. She was really excited about the idea of being able to leave the phone at home while running while still having access to things like a GPS logger and her calendar.
Then I pointed out that would only work if she only ran in like a 40 foot bubble around her phone. (And that we don't know what happens without the phone and what the range really is.) The discovery that the watch doesn't have a GPS in it basically killed her interest in it. If she has to carry the phone anyway, why bother with the watch?
But you're right, if the Apple Watch was basically a tiny phone you could use from your wrist without requiring you to also be carrying the phone, that could be a great product for runners. Hell, if it had a GPS and just synced via your phone while it was in range, that would probably be "good enough" for a ton of runners.
As it is, its dependence on the iPhone for GPS and the fact that we don't know exactly how well it degrades without the phone basically kills its usefulness for runners.
And, of course, for those of us who don't go running and instead spend all our time sitting on our fat asses, I still haven't figured out a reason why we'd want an Apple Watch.