[JC5]

Technically speaking, it's not really necessary. It does help to validate database consistency, because the sum of all transactions is zero.

It also prevents you (from an architectural perspective) to pull money out of thin air. If you create an account with 1000 on it from the start, there has to be a source for that money. That account is hidden and invisible, but will have a balance of -1000. It may look like nothing, but it helps to keep things in check.