It gives major advantage. If you can manage to stay behind your enemy they won't be able to hit you, melee or otherwise. If a warrior keeps keyboard turning and you stay behind him, you take 0 damage he takes full damage, you win. If a caster is casting while you smack his bitch up, and you manage to get behind him faster than he can turn with, his casttime is wasted since he's facing the wrong direction.
Pretty much the same in the less realistic shooter (i.e. Quake), moving target = harder to hit, and you don't lose anything by moving granted you know how to aim.