The auto-brightness features adjusts the light on the screen based on the light intensity of your environment. If your in a dark room, the brightness is dimmed, if in a light room, it is much brighter. Turning the auto brightness feature off would give you the option of having a consistent brightness that you desire, so if you turned it off you could set it at a brightness that you would always want to watch a video at. Hope that helped
It wouldn't give you any more options to change the brightness DURING a video though..