Well, to entice folks to buy 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?
So that you may feel guilty when you judge something on appearance. Usually this involves something that would generally bring contempt upon itself.
Think about the next time you are blithely told that you can't judge.