Currently the 3rd person camera does not have the ability to bank or pitch with the airplanes, it always seems to be stabilized towards the airplanes general direction of travel.
The option, when turned on, would make it so that the plane, relative to the camera, is always in the same position. The best way to explain it would be to watch this (http://www.youtube.com/watch?v=SsCGi4FjwkU) H.A.W.X. video where the camera behaves in the way that is being proposed by this optional feature.
While the current camera is good for soldiers (that do not bank or pitch), ground vehicles, sea vehicles and ok for helicopters, I feel that the third person camera for airplanes could be improved with this toggleable option.