First of all, there's obviously great standups who stand at the mic and never move much. But standup also gives one the opportunity to use the space of the stage, to gesticulate wildly or subtly, to pace around, to be visually interesting. Yet I never hear comics discuss this aspect of standup. It may be because prop comics and Dane Cook types have made other comics embarrassed to admit that standup is, whether you like it or not, a very visual medium. If it weren't, the audience wouldn't keep their eyes focused on the performer. I always appreciate a comic who makes the space of the stage an interesting place. To me, the performance seems bigger if a comic treats the entire stage as if it were his natural habitat. It's something the best comics (with obvious exceptions) do very well.
I'm still at the point where I can't walk and chew gum at the same time on stage. I try to be aware of what I'm doing physically and remember to move around a bit, but I can't say that I'm able to do it un-self-consciously.
I think the main value to moving around a bit is simply that it's more interesting to watch, particularly over the course of a long performance. One reason (among many others) open mics are dull to witness is because comic after comic tends to just stand there. Your eyes get bored.