The claim is that it is only part of the evolution of our hands. Like an ape, we can grab and hold onto things, but the tightness we create with our fist is unique to us. There has to be some reason that we differ from the animals in this manner, and this is the best theory I've heard.
To answer your earlier post: it's not always about 'survival' though. Punching, and fighting in general, is more of a dominance issue than a survival one, even today. I'm obviously no sort of expert in the matter, but I can easily envision our less evolved selves fighting each other for the sake of tribal and personal dominance.
No one is saying this is set in stone, it just looks likely that it is the best explanation out there. However I do see a problem with it. Women's hands have evolved the same way, which would have to mean that at some point in our distant past both women and men were fighters. Which goes slightly against what we know about male dominance and subjugation of the prettier half of the species.