This is typically for offline payments like (bank transfers or cheques (checks)) where a client submits one payment to pay for more than one invoice.

Idea: When I record a payment that exceeds the balance of invoice#1 I am prompted to apply the difference to another invoice or as a credit to be applied when I record a payment for invoice #2. Ideally I prefer to see a record of the total payment amount for auditing purposes.

The "credit" processing could also help in cases where a customer prepays for work before the invoice is applied or a credit on the account is offered (instead of a refund) that can be applied on future billing.