As the above post says, the Apple Watch is an accessory. It doesn't do much that you can't already do on an iPhone. However, it much more
convenient for doing a lot of things than an iPhone. This article explains it well.
How is the Apple Watch doing? - Benedict Evans
In my experience with Apple Watch and from reading what other people use it for the most useful functions are:
Notifications - Quick and quiet with a tap on your wrist. It takes one second to glance at, and if it's not important you are done. No pulling your phone out or running across the room to grab your iPhone just to discover some marketing email.
Quick Info - It is in fact a watch! It tells you the time and date and has a timer, alarm and stopwatch. But it also provides quick access to weather, calendar, todo list, music controls, etc. right on your wrist.
Messaging - You probably rarely use Siri to dictate messages on your iPhone. But you'll be surprised with how useful and easy it is to reply to messages with Siri on Apple Watch. No, I don't use it when I'm in a loud public place. But if I'm at home or in my office or driving, I use Siri to reply all the time. I think this is by far the most underestimated feature of the Apple Watch.
Activity/Workout - The usefulness of the will vary more between different people. Since I am usually sitting at a desk working at a computer, I particularly like the reminder to stand and move around if I have been sitting for too long.
Siri - Just lift your wrist and say "Hey Siri". She can do almost everything she can on iPhone. Tell you sports scores and standings, movie info and showtimes, create reminders, set timers and alarms, do math and unit conversions, and look up random info on Wikipedia.
Apple Pay - Again, this will vary between people depending on the places you shop.
