Under some circumstances, it is not worth charging a user until a minimum amount has been reached. It would be useful in cases where IE the expected charges are small on a month to month basis, or where the user starts a "charged on the first of the month" prorated subscription on the 30th, to allow invoices to be merged until certain criteria are met.

The biggest criteria being total across the invoices, but a second optional criteria might be the total length of time that an invoice has been deferred. IE you can defer up to a year, but after that year, everything in the past year will be charged.