Through its complete implementation of all PostScript commands and also the availability of lots of output formats, ghostscript is not only used to display PostScript, but also by many programs working with PostScript in one way or another, e.g. it's used by frame2html and latex2html to extract figures and symbols unknown to HTML.
Together with a decent frontend (such as ghostview or gv) it's also a very handy tool to preview PostScript documents on screen.
When using ghostscript to produce PDF output, the output differs from Adobe's Distiller output in that it is not compressed (no image downsampling is performed) and links are displayed different. It's also one of the know problems that ps2pdf sometimes produces output with missing links.