What, you've never forgotten any of the controls in a game? Never needed to figure out how to find a certain feature or do something specific?
Besides, instruction manuals can be pretty interesting when they actually put a little effort into them. For example, the Manhunt manual. Even run-of-the-mill booklets can be kinda sweet; if you're lucky, you might even get a weapons rundown or some character/plot background and exposition that wasn't mentioned in the game itself.