Sometimes printing to a disk file rather than to a printer can be useful. What does printing to a disk file mean? It means that the same data that normally would be sent to the printer is shunted to a disk file, either locally or on the network. The file isn't a copy of the document you were printing; it contains all the special formatting codes that control the printer. Codes that change fonts, print graphics, set margins, break pages, and add attributes such as underline, bold, and so on are all included in this type of file. Print files destined for PostScript printers typically include a PostScript preamble, too.
The primary use of print-to-disk is to send formatted PostScript files to a service bureau for professional printing. You don't even need to own a PostScript printer to do this.