Covers are massively important for books without a built-in audience. I'll often be browsing an online store for something new, maybe a sci-fi debut or an obscure literary novel, and my thumb just stops scrolling. It's never because I read the blurb first—the image has to arrest me. A cover tells you the genre, the tone, the ambition of the book before you read a single word.
A great example is the recent trend in fantasy: illustrated, almost painterly covers like those for 'The Priory of the Orange Tree' signal a sprawling, epic, traditional feel. A minimalist, stark cover with a single symbol suggests something more literary or thriller-esque. I've bought books solely because the cover promised a specific mood that matched my craving, like a dark, rainy cityscape for a noir.
Of course, a bad cover can be a huge barrier. I've skipped over well-reviewed books because the cover looked self-published or like a generic stock photo collage; it suggests a lack of care that makes me doubt the contents. For established authors, a cover refresh can completely revive interest, too. I almost waited for the paperback of a novel because the hardcover art was so bland, but the new edition looked stunning and I caved.