Most likely, they want to use the SO button to generate the PO. I am in the same situation. The client has a large MASTER PO with the vendor for a specific quantity of an item. The item gets drop shipped. It is important to relieve the Master PO, since they pay different costs for the items depending on how many they shipped already this year. First 10,000 units at $3, next 10,000 at $2.90, etc.