It makes computing accessible. It appeals to our intuitive senses. Where in the real world have you ever seen a WIMP interface? Maybe in your file cabinet, at best. WIMP is like doing everything with buttons and levers whose experience is disconnected from their effects. MPG is like manipulating the world directly. This is why kids get it so quickly, why it's so much more enjoyable than WIMP.
Apple realized this breakthrough, not just with the iPhone but with the iPod, whose controls were already gesture based (albeit with only two gestures). Previous to the iPhone, touch screen phone UIs were just WIMP based interfaces in which a stylus or finger replaced the mouse as pointer.