Don't you know the problem?
1.) You are programming some software that connects to the serial interface of your computer
(e.g. some modem software)
and you want to test your software without any modem (because it's already in use, isn't it?).
So you need two computers and a so called nullmodem with TXD and RXD lines crossed.
But you can do it in a much better way, because you've got a Linux machine:
You can connect two equal instances of your program with this nullmodem utility.
2.) You have a connection problem with Pseudo-Terminals (ptys)
where one program should connect to an existing pty,
but you want to start and stop the program that opens the pty-master.
So you need some kind of an adapter: nullmodem.