I bought a doughnut from a store and they gave me a receipt for the doughnut. I don't need a receipt for the doughnut. Man, I'll just give you money, then you give me the doughnut. End of transaction. We don't need to bring ink and paper into this. I just can't imagine a scenario where I would have to prove that I bought a doughnut. Some skeptical friend: "Don't even act like I didn't get that doughnut, I've got the documentation right here. Oh wait, it's at home, in the file... under D for doughnut.