Long ago I received a rejection that incensed the author, but gave me new perspectives on evaluating a book. The editor had liked the book enough to bring it up to her editorial board, but in the end one of the reasons they gave for rejection was because they couldn’t picture the cover.
Whether we’ll admit to it or not, we do judge a book on the cover. Thriller readers will pass on a cozy mystery because it’s not the sort of cover (style, type of book) they tend to read. We would never put a romance clinch cover on a middle grade fantasy or a science fiction cover on a thriller.
When we’re browsing the bookshelves at Barnes & Noble we look at titles and covers first. They give us insight into the book. They hint at the genre, the hook and even the tone of the writing. It’s why your agent will fight so hard against a cover she doesn’t like, or doesn’t feel tells enough of a story.
When writing your story, and your query, it’s not a bad idea to picture the cover. In the case of the author above, I think (if I remember correctly) she was a wonderful writer, but there wasn’t enough of a hook there for them to figure out how the book was going to stand out. There wasn’t enough to build a cover off of.
The cover needs to tell a story, just as much as the book itself tells a story. This doesn’t mean the cover artist can’t take creative license (it often does), but it should be exciting enough to jump off the shelf and into the hands of the reader. It should be something the reader notices more than the other books next to it.