I knew you could get notifications to it and that made checking them faster, but I didnt realise that was likely the main reason for having one.
I'm trialling one as a gift, so I didn't pay as close attention to what it does as I would for other products
In addition to activity, mentioned above, it works great with Siri. Instead of getting your phone out, it can set reminders, add stuff to your calendar, ask it to check a sports score, set an alarm, work as a calculator, etc.
One of the main things I use it for is to control music on my iPhone. I use Apple Music and you can just tell it what you want to hear and it will play. The Music app on the watch will also let you control which Airplay device you want to hear music on by tapping the button (if you have Airplay devices). You can also control volume, skip songs, use your Watch without the phone for music via Bluetooth. If you use another app, you don't get the Siri commands, but you can still stop and start podcasts on the iPhone with apps like Overcast and Downcast.
I also use mine to see if I have any important mail. I like to be able to just swipe through junk and delete them without ever getting on my iPhone.
The Carrot app puts the current weather and forecast on my wrist. I always look at it before leaving the house.
If you have Homekit devices it is nice to have on the wrist, as well. I can have my phone parked across the room and just tell my watch to turn on "bedroom" and my lamp comes on.
I also use mine when I grill out. I have an app that uses Bluetooth to connect to the phone to tell it the temp of my grill and the meat on the grill. Using Bluetooth on the watch gives it the ability to double the range since the phone connects to the grill thermometer and the watch connects to the phone.
Those are just a few of the things it can do, in addition to getting notifications...which I love. I have security cameras at the house and it can tell me the minute someone or something sets them off.