Why Is Book Cover Design So Important for Authors?

I’ve been wondering how much a book’s cover really influences reader interest. Can a professionally designed cover make or break a book’s success? For authors, especially those self-publishing, investing in book cover design seems crucial but is it worth the cost? Are there key elements that make a cover more appealing, or does content matter more than visuals? I’d love to hear from experienced authors or designers about how a strong cover can impact sales and reader engagement.