Using Teensy 4.0's Serial, SerialUSB1 and SerialUSB2
In dual/triple serial mode the Teensy implements a composite USB device consisting of 3 CDC-ACM (virtual Serial) devices. In Windows they will show as 3 different serial ports with dedicated COM Port numbers.
TyCommnander (Alternative uploader) handles these modes nicely and opens a dedicated serial monitor for each of the ports. (it is generally much more useful as the stock uploader)
As for your question about the use cases: The multiple serial modes are especially useful if you have a PC application which communicates with your Teensy over the main serial port. I.e. it sends commands, receives results etc over this port. If you now want/need some debugging/logging information it is really convenient to use the second port for that and monitor debug messages on a standard terminal (or tyCommander). You can do something like that:
Stream&amp; control = Serial;
Stream&amp; debug = SerialUSB1;
//...
void someFunction()
{
  control.println(someData);                       // send data to the PC over the main communication channel
  debug.printf(&quot;%d sent some data\n&quot;, millis());   // send some debug info on the debug channel
  //....
}

You find more information and example code in the Teensy user WIKI: https://github.com/TeensyUser/doc/wiki/Serial