Its the same on the 747-400. An aft C of G means you can haul more weight and it saves you fuel when airborne.
Purely down to aerodynamics. Very simply
: Stabiliser produces downforce which the wing has to overcome. Stick the weight at the back and the stabiliser has to do less work, which means the wing can produce more net lift.
Recently flew at 744 out of Mexico City. Had to have an aft C of G just to get the aircraft out of there, but it made a substantial difference on the fuel burnt during the cruise.
T'bug
