The Apple Watch is intended to be a companion to your iPhone, not a replacement. Its primary purpose is notifications so that when you get a text message, email or phone call, you can quickly see if is important enough to require your attention without having to pull your phone out of your pocket. It is not just for the rich, as some people think, it is for everyone: Old, young, small, big, rich and poor.
In a single glance you can see if an email is from an important client or just some rubbish Spam meant for the bin. You can see if that text is from your wife/girlfriend/child or an emergency or something you can respond to later. You can see if the person calling is someone you used to speak to now or someone you can call back when you are in a better location or less busy.
If you are the type that always has your iPhone in your hand all the time, then the Watch probably is not much use to you. But if you want a convenient way to see who is texting, emailing or calling without having to dig your phone out of your pocket, and you are willing to shell out $400 plus, then it is a nice thing to have.