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Yes, pad wear spacers are blank plates to take up the gap created by worn pads.
Why? Couple of reasons;
1. As pads wear the pistons become pushed out further. This hinders the 'pull back' of the piston as the concentricity is effected the more it becomes pushed out.
2. For the foul weather customers they keep pistons in the housing longer and don't expose the piston to body area to as much grime. Fresh pads do the same thing of course.
3. Maximum wear- this allows one to run the pads down more without the risck of piston binding as the pad becomes worn. On fixed calipers the pads should be replaced at about 30% remaining for max effectiveness. Should the pad begin to taper wear they will wear much quicker and drag. Plates allow for the same overall displacement with better function.