Do judge a book by its cover?

I know we're always told that we shouldn't judge a book by its cover (literally and metaphorically) but, as for the literal sense, why do books have covers?
A cover is meant to display what the book is about in some way. It's meant to catch your attention and shout, "Hey you! Buy me! Read me!"
So if we're meant to ignore a cover why have one?
Do you think it's possible to completely ignore a book cover? Or not judge anything? Or do you think it's human nature to make pre-judgements?
Just wondering what you thought. Let me know.