I've been self-publishing since 2009 and now have seven titles out there. In one of my previous lives, I was a graphic designer in advertising and marketing in NY Metro for a variety of clients. I worked in the field full time for more than twenty years and still work on a more limited scale primarily in publishing and book design. The most important single point to consider is a general marketing mind-set: once a book is available tot he market, it has become a consumer product, and the book's cover, whether on a shelf or in an online listing, is the product packaging. Packaging has very specific jobs to do and a book cover is no different, but the primary goals are to attract YOUR niche reader and get them to open the book up and read. That should motivate them to buy the book.
I agree and have been fortunate to find some pre-made covers that were suitable for my novels because the cost of bespoke covers would have exceed sales for the first two years.
I think all readers, and authors of course, are first attracted by a book's cover and title (and author's name if it's known), which takes the reader to information about the book, bio, reviews, etc. For the ebook reader, based on my own selection process, it's rare to sneak a peek inside. I am fortunate to have an artist wife who helps in the cover creation process.