The reasons to use a link dereferer service are different but the most common reason is just to avoid that the website where the visitors are coming from will appear inside counter statistics or server logs.
Visit this website without a dereferer: http://www.whatismyreferer.com/
Now visit the same website with dereferer: http://xww.ro/?http://www.whatismyreferer.com/
This mechanism prevents the website where you came from appearing on counter statitics or log files.
To derefer all the external links on your forum, blog or homepage using xww.ro, just add this script somewhere in your html code: