Covers are an important thing, otherwise Japanese publishers wouldn't put so much effort into them. A good cover can make the sales go up a little bit, principally for people who don't buy the magazine and see the manga on the bookstore, it is the first invitation card for many people in a market huge like manga market in Japan. I'm doing this very often with Volume Cover Thread here and Amazon, I pick what interests me more from the cover, I don't have time to read all series that appears in front of me, it is too much, all the more to Japanese readers.