Just going to make a few assumptions. Apple has a gamification system similar to fitbit. You complete rings (goals) by walking, standing up and exercising. Except you are competing against yourself and they prompt you with daily reminders / notifications. Being that Apple Watch is more than a notification system, it's a bit more difficult to ignore. Let's say you're on a 80 day streak completing 100% of your goals of standing up, exercising and walking. You'll want to keep going. At 24 hour hackathon, there was someone trying to keep their streak up so they ended up leaving to jog around the block to complete their daily activity.
Of course, this isn't to say that it can't be possible without the Apple Watch. It's just that it's more difficult to ignore.
You can now do Activity Sharing so it can be more than competing with just yourself if you have friends who use a an Apple Watch. It can be a great motivational tool if you use it as such.
Of course, this isn't to say that it can't be possible without the Apple Watch. It's just that it's more difficult to ignore.