Dealer prep fees = Something the dealer can put a name on and charge extra.
Like when we once asked the mechanic how much to change oil, and he said "twenty bucks" and when we were done, we handed him a twenty and walked away with an oil change.
NOW, we ask the mechanic's desk-jockey (can't talk to mechanic anymore) and he says "oil change, twenty bucks" ... and when you get the bill, it says "oil change, $20 bucks, oil disposal $2, shop rags, $2 bucks ... so they label the charges that should be included in the Shop Rates of $120 an hour or more and add it to the bill as if we suddenly became idiots overnight.
Add profit just because it can have a name / label, sorta.