The problem is that you cant do anything with the iPhone while it is in your pocket, thats the main ****ing problem. You cant see what notification you just got, if it is an mail that need an urgent response or if it is a notification that someone liked your facebook status. You cant change the song your are listening to and see what is the next one. You cant send a short voice msg to anyone to say for example "Hey, Im coming" or "Ill be late 5 min". In general, your iPhone is useless while it is in your pocket. The Apple Watch is an extensión to your iPhone and everything you do with it. It is on your wrist and feels so much more natural and personal just to tap your wrist to perform a certain simple task instead of taking your iPhone out and start typing.
Ask yourself: How many times a day you pull your iPhone just to see the hour or what notification you just got, to switch a song, to post a tweet (if any). Docens? Hundreds? The Apple watch allows you to do the above and so much more in a easy and non obstructive way.
How many calls you missed or how many msg you responded late just because your iPhone is in the other room and your didnt hear it? How many times you didnt hear the vibration when your iPhone was silenced? None of this would ever happen if you wear the watch.
The opportunity this technology is giving is enourmos and just like Tim said, I cant wait to see what the developers would come out with.