The expression ‘You can’t judge a book by its cover’ is right. It is not a good idea to prejudge the value of something or even someone simply by outward appearances.
But being humans, we like to make quick assessments. The Caveman probably did not think, ‘Does that tiger really look hungry or is he just curious.’ He judged the book by the cover, tiger stripes.
So, book covers are important. They create a brand for the title and draws our attention. It is the first, maybe not the fairest way, to judge a book.