I just stumbled across a great article on Boston.com about "Embodied Cognition". It describes how we often perform complex cognitive tasks better if we are allowed to move around. This reminded me a lot of an article called "The evolution of goal-directed cognition" in which Thomas Hills describes how complex cognition might have evolved from simpler foraging behaviors. There is too much in these two articles that make sense for a single blog post, but I'm currently in the process of writing an article along these lines and I'll try to incorporate these thoughts into it.
Our brain is an active organ which constantly produces activity, even when we seem to be doing nothing: we either do something or we're planning to do something or we imagine doing something. And apparently acting on this urge to be active helps many other brain functions, too, presumably because these functions evolved from moving around.