As those who play Pokémon Go know, the current game requires you to keep your phone out with the app running in order to detect nearby Pokémon, trigger PokéStops, and accumulate egg-hatching kilometers as you walk. This is a battery drainer and a bit of a pain.
With Pokémon Go on your wrist, you can do all of the above with your phone in your pocket. Smart. The only thing that requires your iPhone is the actual act of catching a Pokémon, something that happens occasionally, rather than continuously.
Great partnership here. One that will solidify the existing Pokémon Go base, bring new players, and sell some Apple Watches. Smart.