Give an agent a task and it will tell you it is done. That is the problem. "Done" is a claim, not a fact.
A receipt turns the claim into a fact. Every action an agent takes writes a signed record: who acted, what they did, what they touched, and nothing more. You stop trusting the story and start reading the ledger.
This is the difference between a demo and something a bank, a government, or a serious business can actually run. Not more intelligence. More proof.