THe null modem thing is partly correct. The rest are used for hardware flow control and handshaking. Hardware flow (and I guess hardware handshaking) control is not always used (just takes more work and doesn't seem to be needed usually). Do I need to explain flow control or handshaking?
On your wiki link, it mentions:
"The handshaking and control lines of the interface are intended for the setup and takedown of a dial-up communication circuit; in particular, the use of handshake lines for flow control is not reliably implemented in many devices. "