[Note: This is beyond my range of expertise.] Apparently it's useful to somebody in the case where the Printer cannot communicate with the output device. In this case, the Printer cannot be shut down with synchronization. However, the Printer can be shut down without synchronization, which allows any previously hung operations to complete.
>
> I think that many printers are likely to implement synchronization by
> counting the number of sheets that have left the printer. In that case they
> would prefer to always do a NPRO when shutting down, otherwise they would be
> left with a number of unfinished sheets in the printer which complicates the
> synchronization after a restart. Also, many printers, including web fed
> printers, have to do a NPRO otherwise they will spoil the sheets that are
> left in the machine.
Hmmm. In our case, on Shutdown, "non-process-run-out"='true' is only valid with "synchronize"='true'.
> ...
-Carl