The purpose of this is to track the item state. If you order more than one of something, a partial shipment could occur. After shipment, a partial return might also occur. The alternative is to complicate the data model. You would have to split the item into two rows each time a partial state transition occured. I once posted that you could bring an ecommerce site to a halt by ordering a huge number of any item, actually just placing a huge number of any item in your basket, which doesn't even require a user account.
In the merchant-system package I am working on, I use a single row until a split is needed. I would rather handle the complexity in one place instead of on every page of the checkout system as in ecommerce.
