The handle conveniently located above the window used to stabilize of oneself during turbulant driving. Can be found in most vehicles above passanger windows, and sometimes above the drivers' for extra convenience. Most often used in off-road environments, such as mudding, or auto-crossing.
Ian - "let's see if we can make it through this trench at 35 mph.."
Charlie - "OH SHIT" (reaches for handle and braces for impact)