Well that’s just a saying about how you can never be private on Internet as it’s always possible to digg out who you are in real life by bits and pieces you left about yourself everywhere on the Web.
Reverse domain tool comes handy when you want to know what other websites / domains the server / IP hosts. Because majority of the websites there are in shared hosting plans. Even for dedicated or VPS dudes who have their own unique IP, it’d still need them to host their other domains on the same IP and web server.
Interested in what other websites or domains a man has? If you know just one of his domain or site, you can literally get the others that he own by a single click here: http://www.yougetsignal.com/tools/web-sites-on-web-server/