I can think of many reasons to have one. . . especially for women who carry their iPhones in their purses. Phone call or text comes in. . . and they are forever digging in their purses trying to get their iPhone out to answer or check the text. Most often the call goes to message before they can get it out or they may drop the phone in fumbling for it. With the Apple Watch they don't need to constantly dig for their iPhone. . . They can see the text at a glance or answer the call.
Similarly, guys who keep their iPhones in pockets can merely glance to see if the text is important or answer without digging their phone out of their pocket all the time. There are lots of other functions separate from the iPhone. . . that tie from the iPhone or back to it.
Thefts of iPhones should drop with the use of the Apple Watch. . . the run by and snatch thefts won't happen with the watch being used instead. ApplePay will be easier and more convenient.
And, of course, you can use it as a time telling device with as many complications as you desire.
We have had a Samsung Gear II watch for several months now. I was the one using it to begin with, but it turns out that it is more helpful to my wife. Once you pair it with your Samsung phone, you can answer calls without digging your phone out. You can also read your texts etc. inconspicuously. It has a whole host of useful functions and features. It actually is more useful than one would tend to imagine. I am sure that the Apple watch adds similar function to the iPhone.
My wife is like that. And no, she won't wear a bluetooth earpiece.