Have you ever wondered why you have to provide receipts for reimbursement, despite the fact that the vast majority of people here could put together a forged receipt quite easily?

It's not because the receipt proves anything. It's because if there was ever any question, it gives a validator something to actually validate, to call up the putative place of business and confirm the transaction and the size.

It's one of those deals where the mere threat of being able to validate tends to keep the system humming along fairly well, even to the point that many of the participants have forgotten the justification for the system. Presumably there's someone out there routinely forging receipts for fun and profit but it must not be that big a fraud considered across the entire system.

Same thing here.