Part of it is not having to pull the phone out for simple things like checking notifications, Apple Pay, controlling music playback, etc.

When I’m at home even when the watch is connected to the phone a lot of times my phone isn’t in my pocket and may be in another room, so if I get a call I can quickly glance, see if I want to answer it, and dismiss it if I don’t without having to go get my phone.

I have a cellular watch so I also like not having to take my phone with me sometimes, especially if I go for a run or a bike ride. That way I still can call someone if my bike breaks and I can’t fix it or something like that but I don’t have to carry a bulky phone.