I'm not the type that runs around with a phone permanently glued to my left hand.
As such, the Apple Watch's integration with phone and message notifications means that I don't have to dig around for my phone in order to see if I need to take a call or answer a text.
I've found that my most surprising use case, however, is payments. Wireless Apple Pay is extremely convenient and secure and again, I don't have to dig out my phone while I'm juggling everything else at the register.
If I needed a super-adventure sports watch a Garmin might make sense... but since I don't. having a device that plays well with everything else in my "ecosystem" is the better choice.