The shuttle was designed to glide to a landing, so there's probably some lift generated by its own wings at several hundred knots airspeed. The specific mounting angle (not absolutely parallel to the fuselage top of the 747) is likely to optimize the shuttle's own aerodynamics in this manner.

The V-22 Osprey is able to take-off at higher than its maximum hover weight by angling the nacelles forward about 45 degrees and using a short horizontal run to move air over the rotor discs and increase the lift. As it burns off fuel during a mission it will return within its maximum hover weight for vertical landing (or it can use a run-on landing if still over hover weight).