Tissues and Sex

Why is it that tissues are always found after sex? For example, in Ebichu. Is it because they used it to clean up?

I'm surprised no one has brought this up but, has anyone ever heard of socks? Tissues, especially cheap tissues just break apart on your cock when your trying to wipe up. Then you have to clean up the tissues, and that comes to a painfull annoying 'scrape and pull' technique.