Sometimes you want to add line items to an Invoice on different days and at different times, for a “Running Account” For example, if on day 1 you want to add “Open File” as a line item and on Day 2 you want to add “Consumables” and on Day 7 “Expenses” then you can use an “Open Invoice”
And Open Invoice is only completed and sent to your client when you mark it as “Completed and Issue Invoice”
If you simply “Save” an open Invoice, the Invoice remains open until you are ready to finalise it.