An example might be there is a deal for free shipping if you buy 10 speakers and you only have 8 in your project, or you want to order products for an employee.
To do so, go to the catalog and find the product you want to add to the PO. Click that product and make sure you remember the PO number. From there, go to the upper right by the menu, and click add to order. This will ask you what PO you want to add it to, so remember your PO number
After you select the PO, then you can go to the PO and see the added item in the PO and control the quantity.