In theory, all Internet services should have their own well-known port. Occasionally a service will use a port already designated for use by another service. As you can imagine, this can cause a lot of problems. When a request comes in on that line, either daemon could respond, if both servers were active. You can redirect a telnet request away from the telnet port to another well-known port.
Some services can respond to a telnet request.
Others cannot or do not. This is the port that sends and receives electronic mail. There's no real reason to do this under most circumstances. The point of this, however, is that servers other than the telnet server may know how to handle a telnet request. This makes telnet quite a bit more useful under certain circumstances.