For me: Notifications on the wrist are more convenient than taking out the phone and looking at it. A minute or two ago I got a phone call while I was in the middle of writing a reply to this very forum. I glanced at my wrist, saw it was a number I didn’t recognize, and tapped the REJECT button, all without being interrupted from what I’m doing. I barely had to lift one hand from the keyboard!
Similarly, I also use it to check messages
I also like it as a workout tracker and podcast controller.
I check the weather on it too.
In the car, I use Apple Maps and the Watch for turn-by-turn directions. No more robo-voice interrupting my podcast; the Watch nudges my wrist when it’s time to glance at Maps for my next turn.
Over time, I’m doing more app-checking on the Watch rather than the phone. But honestly, other than the use cases above, it’s often easier to take my phone out of my pocket than it is to fiddle with the Watch.
As for Siri: I have a Series 2 watch and Siri is terrible on it. I’m in no rush to upgrade, however.