On my XF ute it sort of 'lifts' the tail gate so it is level with the tray to provide a flat surface, it also helps to clear the tow ball.
Thats all the advantages I can see.
XF died, Now It's a 6-cyl Cortina!
Rear ended 3 times, 4 close calls.
All while I was stationary, still counting the idiots..