It is written in some answers that "eybrow windows" were helping pilots in "tight turns". How did this work? These tiny windows seem located right above the head, so I would not expect to see anything else than the sky right above through them, regardless if turning or not.
In a (steep) turn, airplane is actually flying partially upwards relative to the cockpit directions. It is simply because the turn is (usually) flown with more-or-less constant altitude, but the airplane is significantly banked, so plane of motion intersects cockpit in the same, bank, angle.
So these windows are located approximately in the direction where you are just going. As long as you see only sky there, everything is fine and as it should be. :) But would there be an another airplane, you really want to see it while there is still time to avoid a collision. Whatever you see through front window during a steep turn is either not on your flight path at all or about to collide with nose in nearest following seconds.