Mostly because, get ready for it, I'm lazy. I never have to pick up my phone or take it out of my pocket to read a text. For many of them, I can text answers with about three keystrokes. I can avoid having to pick up my phone, get my thumb on the sensor just right (or typing in the code), and using both hands to type. The notifications are great as there is a red dot that will tell you if you missed a text or something, or if you have a meeting coming up. The exercise rings are fine, but I wouldn't get the phone for that. The instant weather report is great. In three keystrokes I can find out the best time to golf. Sunset is right on the front, so I know the latest I should start a round. The stop watch is absolutely fantastic for grilliing - You literally just twist your wrist to get an update of how long that food has been on.
There are bunch of things it does that I don't really give a crap about. Oh, and my particular golf app is not that great. It gives me distances to the green, more or less reliably, but my app still requires me to get out my phone at least to score the hole.