We are told not to judge a book by its cover. Publishers spend enormous budgets betting that we will anyway.
Cover design actually matters
A good cover sets tone and expectation before you read a single word. It is marketing, but it is also art.
When it goes wrong
Sometimes a stunning cover hides a forgettable story, and a plain one hides something unforgettable. Neither extreme is rare.
A better rule
Read the first page in the store before you decide. The cover gets you to pick it up, the writing decides if you buy it.