One...I think it's a clearance thing...to be able to open in normal home garages as well as parking garages without hittig the overhead...

Two...when sliding something heavy out, there are no clumbsy latches to snag the bottom of whatever your trying to drag out

Three...to extend the lip out past the rear bumper...

Four....A great place to sit when tailgating...

Not that there is any truth to any of it, but I wondered the same exact thing, and it was the best I could come up with..