The PO dropped the upper control arm from the factory position.
This helps cure the Mustangs poor camber when corning allowing the tires to better grip the road. The "negative wedge kit" allows the upper control arm to be dropped a full 1 3/4", while the upper control arm can only be dropped 1" without the kit.
This is definitely a great handling modification for any street car. Its only drawback would be if you were to convert the car into a serious drag strip car. The lowered control arms would reduce your weight transfer during a launch. Of course, the control arms can still be replaced back into their original holes should the need arise.