When the car is lowered the wheels bow in, requiring adjusting the angle of the strut where it mounts to the strut tower. The stock plates at the top of the tower don't allow enough adjustability if lowering more than an inch or so. The aftermarket ones do. I didn't know much about suspension either, really still don't I guess, but to save money I followed directions and installed both the plates and springs so all I had to pay a shop for was the allignment.
__________________
|