You can use it in situations where pulling a smartphone out of your pocket is impractical. For example:
1. While driving. You can easily take a quick glance at your wrist while your hands are on the wheel and your eyes are on the road. Rummaging around for a smartphone in your pocket is significantly more unsafe. I've saved myself 20 minutes on a couple occasions because my wife IMed me to tell me the place we were meeting up has changed, or that she'd already gone to the grocery store and I didn't need to stop there.
2. While exercising. A lot of people don't like to wear things with pockets while out for a run or cycle.
3. In a meeting. It's less rude and less obvious to glance at your watch than to pull your phone out and start fiddling with it.
4. While cooking. This is a pretty big one: oftentimes your hands are very messy and a call comes in, and you'd miss it if you washed your hands. This is also why half of Apple Watch owners have used their nose as an input device.
5. While carrying things. This is great at the grocery store; I always found having to juggle my bags to grab my phone out of my pocket to be a huge hassle. I can flick through my shopping list without dropping what I'm doing now.
6. When your phone is charging. I'm much more inclined to leave my phone in the charger when I'm home now, because I'll get notifications on my wrist if an important call or message comes in. This in turn frees me up to wear pants that don't have pockets at home, which are often more comfortable.
It's also great in general for screening your notifications so you don't have to take your phone out of your pocket.
I think that the best uses for it will actually come about later, when devs realize they aren't like tiny screens for the phone in your pocket. Rather, they're tiny computers that can deliver information to you in bursts of 1-5 seconds (and from you continuously). How many moments are there where the right information could've led you to make a very different decision? I've encountered several - knowing not to go to the grocery store because your wife has already been, or that the person you're meeting will be 15 minutes late, or that your meetup has changed locations, or that it's about to rain in 10 minutes and so it's time to cut my walk short. I can brainstorm several others: remembering that it's your client's daughter's birthday just as you're about to sell to them, or that the item you're looking at at the store is on sale on Amazon right now, or that your employees think that this presentation you're giving is full of bullshit. What else can devs think of? There are a lot of 1-5 second moments in the day, and historically, whenever computation has reached new situations new $100B businesses get founded.