Title says it all and this is not just a burger question, what about those steaks, chicken breasts, pork chops on the grill?
I believe the goal is to get the best sear possible on each side of the meat. Moving the meat around too much could cause it to cook completely without getting a good sear on the outside.
Clearly this doesn't apply when doing anything "rotisserie" style, as you're trying to do the exact opposite: slowly cook the interior of the meat without burning the outside to a crisp or unevenly.
Also, see this similar question: http://cooking.stackexchange.com/questions/6851/grilling-burgers-flip-once-or-keep-flipping