What it does:
This header enables lite-c to interface to the (9 pin) serial communication ports.
The existing port io header only works with (25 pin) parallel ports.

Why it is needed:
Serial communication is a quick, simple way to interface to slave devices such as:

-Microcontrollers
-Sensors (accelerometer, GPS)
-External LCDs

Using these devices you could make a lightgun or something like that.