If you are building to stock, the entire process makes sense (to us, anyways)
At Finish Production you need to assign a serial number to the Item.
When you post the Sales Order (actually that is when the Item is remove from the Inventory)
DBA needs to know what Item is removing. That is why you need to tell it what serial number is about to be remove.
Now, if you are talking about Non-Inventory Items and/or the Sales Order is link to the work order (SO-N), that is another history.
We don't have that situation, so I can't comment or know how it works, or how it should work.
Kelloggs