There are places in the internet, that requires different DNS settings, protocols, etc. to enter them. I can give you a simple example than TOR.
Search for ‘Open DNS Project’. It's a place where you can register a free domain with specific DNS server. You can't access one of those websites without that specific DNS settings. For example, let's just say I registered a free domain named blah.pirate (the extension is real.) on that service. So in order to you to view the blah.pirate website, you need to switch the DNS settings you use. Otherwise you can't log on to the website.
Same goes with TOR. Except that in TOR, everything is encrypted. Even the domains. TOR is that specific DNS settings you need to access the darknet.