If a cover is very simple and has little to no artwork, more attention is brought to the title and therefore should be one that sounds intriguing enough to get the potential reader to pick the book up off the shelf and read the summary on the back or inside of the book.
Others choose to have a complex image on it and these tend to illustrate a specific scene within the book. Some covers have visuals that don't really tie in at all with the actual story and probably the reason they say you shouldn't judge a book by its cover.
Font and color are also things that will catch a shopper's eye and help them decide in picking it up or not. If the book title written in a flowery font and covered in pink, someone looking for a Gothic romance wouldn't give it a second glance.
The same can go for those who avoid the dark and grim tales. They aren't about to pick up a book when its cover is all black with a pair of blood-soaked fangs. Book covers help readers sort visually what type of story they are looking for.
Book covers can vary from those rare finds where the cover is like a siren's song and you are immediately drawn to it and snatch it up to see if the plot sounds as good as the cover seems to allude to, to the ones that are so vague and dull looking you halfheartedly flip through to see if perhaps it is an ugly duckling book who just doesn't look as good as the rest.
The purpose of book covers is to attract the buyer's attention long enough for them to find the book worth their money. If a book has a lackluster cover it is not necessarily going to do poorly, but if it does not have word of mouth helping it, people are less likely to pick it up.
Below is a video of me citing specific examples of book covers and why they caught my attention. Feel free to watch or ignore, it's up to you.
(P.S. Sorry about the sound, I was trying to be quiet in the morning. You can hear it better with headphones in.)