The reason is quite simple: the preview screen is so large and clear that feedback is instantaneous. If the photo isn't just right, I just tilt the phone and fix it.
I impressed my wife the other day after she inadvertently threw the dog's tennis ball into the rain gutter. Enlisted to retrieve it, I discovered that my lawn chair (the handiest excuse for a ladder) was too short to do more than reach my hands above the gutter. I whipped out the iPhone, pointed it in both directions and snapped this picture.

(yep, it's that clear)
I moved the chair to the appropriate rivet and heroically retrieved the ball. So while my other cameras take outstanding pictures of stuff (kids, lighthouses), my iPhone is the one I use to get things done.