Comment by zie

You are thinking it's duplicating work, but it isn't. You are simply recording the whole transaction in a double entry system. i.e. you know where the money came from and where it went. That's all a double entry system does, records both sides of the transaction.

This means when something goes wrong, you have a chance to figure out what in the world happened.

That's it. If you want to think of it as a state machine, you are storing all the state that happened with the resource you are tracking(usually money). This way when some part of the resource is missing, you can go figure out what happened.

Think about it another way: always record enough of the transaction, that if you were required to sit in a courtroom in front of a jury and explain what happened, you could do so. If you can't do that, you didn't store enough information.