A vendor arrives at the warehouse. You create a warehouse receipt and get all the purchase order lines for that vendor. The vendor forgot one box of one item. You change the "qty to receive" on the corresponding warehouse receipt line. Then you post the receipt of this warehouse document. Since there is an outstanding quantity, the warehouse receipt document is not deleted but the "qty received" on the line is updated.
A week later, the vendor returns to the warehouse. He delivers items for several new purchase orders, but also the one box from the previous receipt. The warehouse worker will not remember that there is an outstanding quantity waiting on one of the existing warehouse documents. He just wants to create a new warehouse receipt and get all outstanding purchase orders (the new ones as well as the old).
I can imagine that the standard solution does fit for certain scenarios. But during a lot of my BC implementations we needed to make a customization to delete the warehouse receipt once it is posted (even if there is an outstanding quantity). Therefore I propose a parameter (for example on the location card).