PS is used primarily when you are in a make-to-order business. This is when the finished good cannot be defined prior to beginning the work or it is highly variable.
The WBS structure holds the costs and revenues and provides the vehicle to analyze profitability. In this business environment the revenue is determined and billed on a Sales Document but it is passed to the assigned WBS for reporting.
Materials assigned to a PS structure create requirements that are fed to MRP where PP or Purchasing takes over to produce those materials. Upon completion they are issued to the PS object and their costs are now included in the project.
If you have a predefined finished product with associated Bill of Materials then you really don't need PS. PP would be a better fit as long as the work is repeatable. This would be more of a make-to-stock environment.
What underlying business requirement are you trying to fulfill with the Finished Good?