Hard as it may be to imagine, having two hands with opposable thumbs can actually be pretty restrictive. For one thing, everybody knows that you can do stuff with your hands. No one in their right mind is going to do something like point a gun at a person's arm, because you're pretty much just begging them to grab a bullet out of thin air and throw it at you. So once a character is sufficiently strong, he needs a new way to catch an opponent off guard, and that means giving him a third arm. And what better place to have a third arm than with those shiny white things in your mouth?