Depending on the particular coating, that's not necessarily true. Certainly for the visible scratches and whatnot it would be. But take the swirling. If the index of refraction of the cquartz is close to that of the clear coat that the swirling exists in, and if the coating does a good job filling in those micro scratches, the end result will be a much better looking coat.
Same principal behind why your car looks great wet, then all of a sudden after you totally dry it the swirling shows up.
That said, I'd like to see a head to head of finest vs $60. If it really is mainly the prep that leads to OP's final result, it's almost certainly worth not DIYing.