I haven't had to manage printers in a while, but with some of the older HP printers, the only way to send data to the print engine was via print jobs.
While the printers came with fancy network interface cards with support for almost every network printing protocol you can think of, these were essentially separate devices. The NIC could be used with a number of different models of printer, and the printer would function if you removed the NIC. Without the NIC, the only methods of input were the parallel port and the buttons on the control panel.
If you wanted to upgrade the print engine's firmware (as opposed to the NIC's firmware), it needed to be as a print job. You could submit this job via the parallel port or via the NIC -- it would look the same to the print engine.
I wouldn't be surprised if they could improve things these days where networking is integrated into the printers better, but there is probably a lot of legacy code in the printers.