There are several reasons for using other than sales receipts.

You already mentioned check, money order, charge CC later. We don't charge an order until it's ready to ship, the final charge can be more or less than the original depending on availability or if the customer makes changes.
Need to track backorders. Orders may ship in several parts from one or more locations.


Anyway, another question, is it possible to flag some orders so they are not imported?
The reason for this being sales orders or estimates is not currently supported, when sales orders are needed, we'd need to enter these manually.

Thanks.