For example, one of the Menu Books (number 6) requires you to tune a '65 Mini to 360pp. Of course, you would've collected one in Menu Book 3 by winning one of the Sunday Cup races. But if you had already sold that '65 Mini, you would have to wait for another to appear in the used car dealership rotation - bringing your game progression to a halt for possibly weeks.
I can see the angry reddit posts already of people accidently selling their Ford Mustang Boss 429 ‘69 from Menu Book 14 BEFORE the collection is finished, then having to wait months for it to appear again in the Legendary Dealership.