Book by it's cover

Why do people always say not to judge a book by it's cover? Then what exactly is the cover for. It's no secret most people are visual. The cover is the first thing we see. It determines rather we are going to pick up the book or pick up the one next to it and flip it over and read the synopsis, am I right? A good book cover will make you stop in your tracks JUST to look at it. It will make you ask yourself, "hmm I wonder what that's about?"
Telling people to not judge a book by it's cover is like saying looks don't matter. puh-leaze. They may not be the only thing but like look a book is the FIRST thing. If the packaging is lame what else will make you pick it up?