Until a smartwatch can act as a smartphone I don't get why anyone would find it a need.
Currently the apple watch requires an iphone on your person why wouldn't I just pull my smartphone out of my pocket to do the same function?
Because it's rude to constantly be pulling your phone out to check your messages -- and because your phone instantly assails you with many other non critical notifications.
Or because your phone is in your briefcase and you are in a crowded train. Or your phone is in your office and you are in the conference room next door?
My sense (since I don't have an apple watch yet) is that it would provide notification of only the most critical of messages, letting you forget the rest until you have time or inclination to deal with them. That would be my hope at least.