Tor (short for The Onion Router) is a system intended to enable online anonymity. The Tor client software directs internet traffic through a worldwide volunteer network of servers to conceal a user's location or usage from anyone conducting network surveillance or traffic analysis. Using Tor makes it more difficult to trace Internet activity, including "visits to Web sites, online posts, instant messages and other communication forms", back to the user and is intended to protect users' personal freedom, privacy, and ability to conduct confidential business by keeping their internet activities from being monitored.
So obviously, TOR needs a large amount of servers and as TOR network is not a commercial organisation, it requires voluntary supporters. You can be one of them and turn your Raspberry in a TOR relay.