Man, I used to think covers were just decoration, but my last launch proved me totally wrong. Had this scifi story that was doing okay in my newsletter, but I slapped on a cheap cover I made myself—generic spaceship stock photo, basic font. It just sat there. Then I swapped it for a professional one with a distinct color palette and a character silhouette facing a weird horizon, kept the title font but made it bolder. Sales tripled in a month on KDP. The algorithm didn't change, the blurb was the same. It was purely the shelf appeal. People judge a digital book by its cover faster than a physical one because they're scrolling.
Thing is, a good cover maker doesn't just make it 'pretty.' They understand genre codes. A cozy mystery needs those warm colors and maybe a cat or a teapot; a dark fantasy romance needs that clinch pose and a moody, misty background. If your cover looks like it's in the wrong section, readers bounce. I've seen brilliant literary novels with awful, clip-art-style covers sell nothing, while a pulpy thriller with a sharp, modern typographic cover cleans up. It's the first and sometimes only marketing asset you have.