Not all Dark Web sites use Tor. Some use similar services such as I2P, for example the Silk Road Reloaded. But the principle remains the same. The visitor has to use the same encryption tool as the site and - crucially - know where to find the site, in order to type in the URL and visit.
Infamous examples of Dark Web sites include the Silk Road and its offspring. The Silk Road was (and maybe still is) a website for the buying and selling of recreational drugs. But there are also legitimate uses for the Dark Web. (Also see: Is it legal to buy drugs online?)
People operating within closed, totalitarian societies can use the Dark Web to communicate with the outside world. And given recent revelations about US- and UK government snooping on web use, you may feel it is sensible to take your communication on to the Dark Web. (I'll stick to Facebook, but I like the attention.)