The publishing world

It's literally the reason the cover art exists. To entice you to judge it positively and pick up the book for a closer look.
Whenever I hear anyone say that, I conclude it's one of those things that somehow meant something slightly different when the phrase was coined. Either that or we're all just a bunch of goobers.
- Well, adds exist for literally the same reason and you shouldn't trust everything the add says about the product either
- By judging the saying means like call the book shit just based on the picture on the front. Basically "don't judge before reading", as in before you actually know the thing you're judging.

...The first?
- I wonder if they meant front cover or if first cover is another name for it or the name for it in some other language

I mean... just gonna go out on a limb here and guess that the expression exists because within the history of every book ever released, there would be some fantastic books with absolute stinkin' covers.
Yes, covers are there to help you judge a book. But if that was the only metric you used, you would miss out on some good reads.