They say you can't judge a book by its cover. Then why bother to put a cover on one?
Well, to entice folks to read it. That's why there will be photos or a painting or fancy script on a book's cover. Writers and editors and publishers want the cover to sway your judgment. So, then, the axiom actually means nothing useful. So why employ it?
Think about the next time you are blithely told that you can't judge. The old axon actually works against itself.