This spreads out the weight of your torso, and gives ME more balance. I don't feel nearly as in control when my arms are at my side. You must maintain close visual concentration on the trail ahead of you. I try to keep my eyes scanning along the trail from immediately in front of my feet to several strides ahead. But in extremely rocky terrain, I focus on where to place each footplant, and thus avoid faceplants.
Good eyesight is very helpful, but it takes a lot of mental as well as visual focus.
It's just another way in which ultrarunning is predominantly a mental activity. No blisters whatsoever. The best strategy for running in lots of water was to get right in it and have fun. In the first mile of trail I tried to dance around and keep my feet dry - a colossal waste of energy. The traction on the slippery mud was ugly.