A cover entices the reader to take a closer look, but does not make the "sale" itself.
Good covers are like good trailers. They are supposed to draw attention first and foremost, telling the reader "look how cool I am, imagine what's inside".
Imagination alone was all well and good in the past. However, we now live in the digital era, where information overload is the daily news. These days, a good cover can, at best, hope to persuade people to pick up the book, read the blurb, and perhaps read the first few (or random) pages. If the cover manages that, then it has done its job. The rest is the purview of other marketing elements.
Of course, a "good cover" means an amalgam of three components: catchy/intriguing/epic title, enticing/rich/subversive image composition, and a smooth juxtaposition between the two (aka design).
Regards,
Sagacious