As Andy notes, you haven't really posted a complete question - you forgot most of the background material.

Anyway, if you happens to be thinking about a MAX232 or similar transceiver chip: RS232 signals is swinging between positive and negative voltage levels, so the transceiver contains a voltage doubler and a voltage inverter. Because of this, it needs a capacitor to store the negative voltage. It wold have been more obvious with old RS232 transceiver chips, since they required +/-12V or higher supply voltages. Today, most computer electronics that needs negative voltages creates these voltages themselves.

Now obviously, I could have wasted my time writing the above answer, since it isn't obvious if you where thinking about a RS232 transceiver chip. The magic word here is "thinking". We do not know what you are thinking about. We only know the information you took your time to write down in your post. And you did not take much time, so you might not have been too interested in any answer.