Niantic is taking full advantage of the Apple Watch's functionality and incorporating its fitness elements into the already-active game. Apple Watch sessions will count toward hatcing Pokemon Eggs, and gameplay counts towards your personal activity rings. Push notifications can tell you when there's nearby Pokemon or a PokeStop, your eggs hatch, and when you get a medal.
I stopped play Pokemon Go a while ago when I grew frustrated with some of the game's setbacks (and living in a very rural area doesn't help), but if I used an Apple Watch this would certainly bring me back. It sounds like the definitive version of the game, at least so far, and I hope Niantic can build on it.