This type of port is quite slow, and transmits the data signals to external devices. It is often used to connect modems and Personal Digital Assistants (PDA) to a computer.
Serial Cable to connect the port to a device.
The main advantage of serial transmission is that the cables can be almost as long as you wish them to be, because there is very little signal loss over distance.
This type of port is being used much less frequently, because most devices now use Universal Serial Bus ports USB. In fact, there are now some computers that are being made without a standard serial port.