For software that is not WYSIWYG a separate previewer program allows the user to see the final result before committing it to paper. This saves time and even more importantly, saves money and avoids waste. This is particularly important for colour printing where the paper can be very expensive. You must develop the good habit of previewing documents and only printing them when really necessary.
Some previewers are specific to a particular documentation software, for example XDVI is a TEX previewer; it can only preview TEXand LATEXoutput. Other previewers require only that the file is written in the right language, for example a Postscript previewer can display any document file written in Postscript.