When a sales order is created with the dropshipment option enabled, you are not able to change any quantities on either the sales or purchasing order without deleting anything.
I some scenerio's it would be highly favorable to be able to change the quantity and doing so would update the PO and/or SO. This would be useful in the following case:
Customer 1000 request 1000kg of product A, a dropshipment PO is created at the vendor 2000. but due to packages variations, the vendor one time ships 950kg and a different time it could be 1050kg. The Vendor charges by the kg, and you charge the customer also by the kg.
So when the vendor ships 900kg, the sales orders will still say 1000kg, but the order is completely fulfilled and closing the SO would be deleting the dropshipment link. This would require quite some actions to complete. If you could update the PO with the actual amount send by the vendor, which would update the SO