Think about it from the side of the creditor (the person to whom the debt is owed).

The point is, if the creditor is owed a debt, and receives payment for it, and A) He has no reason to think the payment was a mistake at the time he received it, B) He did not falsely induce the debtor to make the payment, then the creditor should not have to worry that the debtor will come to him the next day and say “Uh, that was a mistake, I want the money I owe you back.”