This is part of the compiler, not part of the scala standard library. Its goal is to implement server communication for fsc (fast scala compiler). When you compile with fsc, the daemon server process starts. This is done by running scala.tools.nsc.StandardCompilerServer, which is a subclass scala.tools.util.SocketServer. After starting the fsc server, you avoid the overhead of starting a new JVM instance with every compilation that you perform, which happens when you use scalacinstead fscto compile your code.