Postscript is a common denominator for a number of printers and image producing equipment. If your equipment is PS compatible you can print a page 100% identical with the product comming out of the photosetter. PS uses a number of standardized fonts (like Palatino and Helvetica, which I use in my booklets).
One of the advantages is that PS works independently from the resolution. A print job from a 600 dpi HP4M laser printer can thus be sent to any other printer and possibly be printed with 2400 dpi.
Many laser printers are available today with built-in Postscript at a reasonable price. If you do not have a PS printer you can install a PS driver in Windows 95 and then print in PS format (I did that with my first booklet with the expected result). Steve Jobs attempted in his NeXT computers to make Postscript into a screen language.
Please also see Acrobat and PDF.