This does two things, first, it makes more room for more steering angle. Second, a wider track is more responsive and reduces understeer on turn in and makes the front less likely to wash out in drift. Time for me to invest in some wider fenders!
The plate pushes the upper strut mount outboard and is highly adjustable for caster. The plate's job is to control kingpin inclination angle and caster so you don't have excessive wheel tilt at big steering angles. This can also cause the front end to wash out in drift due to a loss in tire contact patch.
Camber adjustment is mostly made at the strut mounting point and through the lower arm bearing adjustment.