A way to differentiate the appeal of a book on the shelf. Make it pop. Make it stand out amongst the others. Then they figured out what to add to it.
As there are so many of them, and it’s a standard for any book these days, it doesn’t actually differentiate all that much. In a sea of books, at Barnes and Noble or the library, it’s hard to stand out on the sleeve alone, though attempts are still made.
When the work you do becomes commonplace, it’s no longer an innovation, it’s a commodity. It becomes part of the default requirements needed to ship. While that’s not a bad thing by default, it means you should be looking at the next level of complexity. Surely, you can make the book sleeve the best it can be for a project if you’re an author, but that’s likely not enough. Something that serves the audience better in the journey you are trying to take them on is a great thing to thing about when trying to achieve this.
This analogy extends far beyond authors into almost every field imaginable, you just have to know an industry enough to draw the metaphors.