I've played guitar for 20yrs now. Professionally, touring the country and up in Canada, or even as a hobby around the house since it's always been such a part of me.
I still cannot figure out why I make faces, or why anyone else does for that matter. I just "do." Perhaps its due to putting all your control in your fingers/hands and thought process, that all other movements just happen? I do not know.
But some do, and some do more than others. Look at Danny Gatton, the guy could play circles around 99% of the people out there and there is barely a grimace on his face. Then you have your local wanker at the jam session down at the corner bar, and he's rocking all over the place like a mechanical bull is bucking him off!!!
Some people "get it," and some people don't. I've seen guys that have great "licks" and the like, but they are stiff as all hell when those licks come from their fingers. And guys that are too relaxed onstage but for whatever reason they just don't come across to their audiences.