So - somewhere or other, again, I heard the expression "don't judge a book by its cover!"
Unbidden, the thought popped into my head. "Why not?"
Much like Mike Rowe likes to go off on "work smarter not harder" as not only trite, but actively harmful, I realized that while there was a kernel of truth buried in there, it had long ago been overused unto pointlessness.
The question comes down to a very simple one - what are you judging?
I'll grant you this. A cool cover on a book does not mean it was written in a way that you, or I, as readers will find to our individual tastes.
Nevertheless, the cover does serve to tell us many things.
It has the author's name on it. If we're familiar with the author, we can already start forming an opinion on how likely we are to enjoy it, and what genre it may or may not be in depending on what the author writes.
It has a title. If that's chosen reasonably well, it tells us something about the story, its theme, or its tone.
It has a cover image. The details of this image may not match, in any particular, a scene,person, or anything else in the story. Nevertheless, if done well, should evoke the mood of the story, and give us some indication of whether or not spaceships, swords, rayguns, monsters, detectives, druggies, cops, superheroes, or pretentious philosophers are involved.
It will have blurbs.
All of this will be carefully constructed by the publisher in order to get you to look at the book, get interested in the book, buy the book, and read the book.
Ultimately, they want you to come back and buy more, so they want to avoid a total disconnect about what the jacket promises for genre, tone, and topic, and what is inside the book.
In short - you don't know before you read the book if the story is actually any good, but you can get a lot of information about what kind of story it is, and what the publisher thought was important for you to know about it.
This applies to people as well. You can't always tell deep things about a persons character, or all of their depth and likes and dislikes, but you can tell a lot from how they dress, how they present themselves, how they speak, and body language.
You may not know everything about them, and first impressions may always be wrong, but the "cover" of how a person chooses to present themselves does tell you a lot about them.