They say covers are vital for selling books. Any books, anywhere, in whatever format. I suppose if you hit on a good one by accident that's great, but since it's such a subjective thing, how do we know what makes a good cover? And is a good cover a cover that will sell numbers of books? Well, in this case, let's say yes.
Those people in the know say that the average person in a bookshop will decide to buy the book within 10-20 seconds. (I can't say I'm one of them, but I don't disbelieve what they claim) It means the cover has a lot to do with it, and the blurb backs it up. So the cover must be eye-catching, and resonate with something within the average reader to get them to pick it up in the first place. Then the blurb must be short and punchy and relevant - ie give a snapshot idea of what the book is about. (And that is so much easier to say than to achieve!)
Publishers, it is claimed, are appealing to something aspirational in readers, too. They want to be identified with the contents of the book, or to be more precise, what they think the contents might be. They say a good cover alone can sell a book. Indeed the reverse might be true - that readers would buy a book in brown paper if they wanted the story enough. I don't suppose the Potter fans would have refused to buy the latest installment because they didn't like the cover.