What does CallingThreadDispatcher do?

I came across this code and wonder what CallingThreadDispatcher does. I can comment out

val dispatcherId = CallingThreadDispatcher.Id
val props = Props[Greeter].withDispatcher(dispatcherId)

and this test will still work

class GreeterTest extends TestKit(testSystem)
  with WordSpecLike
  with MustMatchers
  with StopSystemAfterAll {

  "The Greeter" must {
    "say Hello World! when a Greeting(\"World\") is sent to it" in {
      val dispatcherId = CallingThreadDispatcher.Id
      val props = Props[Greeter].withDispatcher(dispatcherId)
      val greeter = system.actorOf(props)
      EventFilter.info(message = "Hello World!", occurrences = 1).intercept {
        greeter ! Greeting("World")
      }
    }
  }
}

object GreeterTest {
  val testSystem = {
    val config = ConfigFactory.parseString("""akka.loggers = [akka.testkit.TestEventListener]""")
    ActorSystem("testSystem", config)
  }
}

So what does CallingThreadDispatcher do? and why do we need it?