Most importantly for the meme, that little gap in the metal provides a focal point for people to stare at and say Well there's your problem! It isn't nearly as interesting as "hand vaguely cupped about the lower third with the pinky extended and then curled back touching the bottom left side while up against my face" (which is the transmission killer on my first generation iPhone. If our connection breaks up, there is no reason to assume it was an accident.)

It seems likely that plain old electrical conductivity is in play here. Some people's skin conducts much better than others and anecdotally tape insulation helps. So that is a different mechanism from most(all?) other phones.