Many print book readers still strongly associate cover art with the story. It helps them catalog and remember the story. Good cover art will catch my attention and lead me to read the book's description, which will lead me to buy it and read it.
Of course, the effectiveness of cover art is in the eye of the beholder. Here's a link to an article about the cover art for The Girl with the Dragon Tattoo. I'm not crazy about this cover, but it's certainly distinctive.
http://www.huffingtonpost.com/2010/0..._n_648859.html
L.J.