They all work on bluetooth...
which means you have to have your "smart" phone in your pocket or nearby for it to work.
When connected you can do just about all the things you can do with the phone, calls, email, texting, music, etc.
There is some advantage, I guess, to taking a call on your wrist vs fishing your phone out of your pocket. It is the same as newer cars where you just push a button on the steering wheel to take a call.
Seems too small for any visual functions. My cell phone is almost too small to be useful for email. I'm not interested at this point