One of the biggest myths about evolution is that every trait, feature and behavior has some purpose. But no, they do not. Many are just spandrels. To get more colloquial, there is a lot of “slop” in evolution and what it produces. The habit of fidgeting doesn’t have to be for anything, or even occur evolutionarily as the result of anything quantifiable directly.

It could just be a consequence of having a nervous system above some level of complexity.