Looks like someone used to run a pirate-rental store out of their basement. I remember quite a few people around where I grew up, who did the exact same thing. You'd cut off the flaps and tape the thing together to make a "divider" for the "crate" you had all your games in and then have cart+manual in the dust sleeve in front of said divider. This way people could easily flip through your games and check out the artwork and screenshots before "renting" the game (Usually a couple of bucks a day). I know a lot of kids did this to make a few extra bucks besides their regular paper routes.
As for OP's question, I'd rather have the manuals so I can play the damn games!