You can think of it as “conservation of value”, so you can’t just create money out of thin air in your payment service (credit), without tying it to some account with a corresponding debit.

This originally was intended to protect against typos; eg write a 10 instead of 100, at the end of the day your ledger needs to balance. In software typos are less likely bit it still provides auditability to prevent a large class of bugs from wiping you out.