You probably heard of houses having curb appeal. People won't bother going inside if the outside is unappealing. Same with books, a book could possibly be the best book ever written, but the writer went cheap and used her five-year-old son's artwork turning away potential readers.
This can be confusing for authors since the Twilight series went big using stock images that other authors bought too. Sometimes the cover doesn't matter too much if word of mouth does the advance promotion. Here's some of my favorite covers.
The greenery, cottage, and fire hint at the mysterious and time travel echoing the tagline.
( Don't you hate picking up a book and thinking it is going to be about pirates because of the pirate on the front and it isn't. I remember these bait and switch covers from VHS tape covers too.)