Because an intriguing cover is going to catch our eye and a boring cover is going to turn us off. Unless someone has recommended it to us or the title really draws us in, we're not going to look further at a book with a boring cover. But a book with a well designed cover with interesting and informative graphics? Yeah, we're going to pick that up and check the back or inside flap to see whether the synopsis lives up to the promise of the picture.
Unfortunately it is rather hard to properly enjoy covers at libraries. Sure they have a few out on display, but never nearly enough. So what do I do? I go to the bookstores where they fully realize the importance of a cover and lay their books out on tables. Then I browse among the lovely spreads of awesome and pick up the books that catch my eye. If it passes cover, title and synoposis test, I write the title down in my notebook and look it up at my library later.
Yes, I do actually buy books. However I'm extremely poor and can't buy all the books I read. That's what libraries are for. However if I read a book and decide I want to reread it a year or two down the road, then I will purchase it. I simply have been burned too many times to buy a book without reading it first. That's why I don't ever weed out my bookshelf. I have chosen its inhabitants carefully and loved them long before I bought them. They're not going anywhere.