Why? Business logic and saving bookkeeping costs.

Just like Magento handles this.

You get an order and then you upgrade it to an invoice. You can also create credit memos on returns.

Benefits:
- you only bookkeep invoices (means less bookkeeping costs and transactions)
- failed orders do not need to end up in bookkeeping
- refunded orders which have not yet been processed into invoice status do not need to enter bookkeeping
- creating a credit memo to refund an invoice, simple business logic to keep track of refunds