Printers are increasingly supporting features such as magnetic card encoding, RFID chip programming, barcode generation, and other higher level functions that are inaccessible from the drawing layer.
Windows (as a particular example) has support for sending raw printer commands intermixed with GDI calls using the Escape API:
Using that API with PASSTHROUGH allows an application to insert raw printer commands into the printer command stream.
It would be nice if QPrintEngine (or some other likely suspect) had a supported method of doing this on each platform. As it is, you are forced to give up QPainters functionality and generate a raw print job in the printers native language if you want to access the smallest of these features.