Let's say you decide to move three smidges to the right and one smidge forward. In the time you do that, your opponent has determine the distance you have just moved, and then move the same amount, just to keep the defensive posture he had in the first place! Thus in practice, fighters within one lunge of each other always move.
Also, moving creates the impression you are moving toward the opponent, which causes fear and panic and may create the perceptual illusion you are getting larger.
It keeps the blood flowing and the muscles elastic. Circulation and breathing are crucial elements to winning a fight. You must keep up your energy constant and keep your body loose and relaxed while working strenuously!
It allows you to assess whether an opponent has become wounded or tired. If you are the aggressor, this is ultimately your goal. If it is a mutual combat, this can foster the psychological state that you are the aggressor.
Another reason is that when you are stationary, your options are few. But when you move, you could be doing any number of things. To react to your movements tires and confuses the enemy. The only time you do not move in combat is when you have anticipated your opponent's motion and he does the moving for you.