The RS232 board is a serial IO board. Serial IO is primarily used for communications.
Communications are usually to:
1. A CRT terminal
3. Printing (although parallel printing is more standard).
4. Connection to another computer system when multi-user S-100 systems started to appear Serial IO was almost always the way to connect multiple terminals to the syste, (this was way before networking).
The big advantage of serial over parallel ports for communications is it is pretty simple and could be enabled with just two wires but four lines were most common.