Black Box Explains... Printer Sharing with Windows.
Unlike the earlier DOS operating systems, Windows® doesnt check to see if the printer is busy at the very beginning of the printing process. Windows will send out data to start a job even if the printer is signaling busy or unavailable. If your print sharer doesnt have a buffer, critical printer-initialization information can be lost before your job is started. Once the initialization information is lost, the printer cannot interpret the job correctly.
A buffered print-sharing device is the most practical solution. When Windows starts printing to a buffered port, it thinks its talking directly to the printer, and the critical initialization information is stored by the buffer. The buffer can send out a busy signal to Windows, so it delays sending more information until the buffer is accessible again.