After looking at the title, your readers will probably look at your cover. Granted, everyone says you shouldn't judge a book by it's cover, but don't we anyway?
It doesn't necessarily have to have words on it, but if your book is about monkeys, it doesn't make sense to have a dog on the cover, now does it?
Having a good cover gives imagination to a reader.
When I look at a cover, I get the mood of the book. Usually, sadder, more serious stories have more lighter colors, while a lot of the humourous stories are jovial and have an abundance of color on them!
Granted, my cover on the right isn't the sexiest cover ever, but hey, I never said this was about monkeys! It's all about making YOU popular!
Sometimes, a good cover displays an emotion or setting that words can't.
Here is what a good cover can tell you:
-Setting
-Mood
-Theme (yes, it is different than mood)
-Main characters
-How someone looks like
-What kind of story is it? (romance, werewolf, humor?)
-Mystery (is your main character(s) hiding something? can it be a symbol to something in the story?)
-Emblem
-Etc.....
All of these things make a story interesting, and attract more readers!