Originally Posted by Steve M

I remember at some point one airline had these installed as an anti-hijacking measure, under the theory that if the airplane was hijacked and landed (such as for re-fueling), the cockpit crew could bail out through the roof hatch to remove the possibility of taking off again from the hijackers' set of options. This was controversial at the time, as people had mixed feelings about the cockpit crew abandoning the ship with hijacked passengers aboard.