Well, like any other smart watch the benefits are:
* Hardly ever need to take the phone out for texts, emails, etc, just glance at the watch. Can even reply with the watch if needed.
* Believe it or not, it can be useful to just glance at your wrist to find out the current time and maybe how much time is left before sunset.
* Phone never makes any noise to annoy everyone around me, the watch just vibrates.
* It's impossible to miss a call/text with the watch vibrations - though of course you can turn that off when you want to, and I only have it buzz for people on the whitelist.
* Replaced my alarm clock - again, can't miss the watch buzzing.
* If you accidentally drop/leave your phone, the watch will warn you. This has saved me twice.
* Smooths a lot of interactions - for instance it's super simple to just use Shazam to ID a song by tapping the watch twice rather than pulling out the phone, unlocking it, swiping to find the app, running the app. Also makes for a great conversation piece at times.
* I find the fitness tracking to be endlessly useful when I go for my walks, hikes, or bike rides. Turns out my pulse rate tells me a lot. And of course actual distance traveled, speed, steps, and altitude changes are really useful. And with the watch you don't even need to bring the phone (though I usually do).
* When biking, voice commands to the watch can be very useful. I have actually gotten myself lost in a new area and just hold down the fob and say 'navigate to [x]' and off it goes with turn by turn instructions.
* Makes paying for things super easy - just tap the watch, no need for hauling a card out of your wallet (which only old people carry now) or phone.
* Works while swimming too, forget using your phone there.
The only thing here that /only/ the watch can do is the fitness tracking - without a smartwatch I'd have to use a fitness band, which is basically a cut down smartwatch. Everything else can of course be done more clumsily without the watch. But it just makes everything faster, easier, less friction, and isn't that the entire dream of tech? Hell you don't /need/ a smartphone, it just makes things much easier. And the watch makes them easier still.
Of course until you've actually experienced all this you can't even imagine how much faster and easier it is doing many things with the watch instead of the phone so there doesn't seem to be a killer feature. I got my first smart watch (Moto 360) just for the texts on my wrist (and the custom faces). The rest I found out later.