As another poster said, it's for servicing.

But it's got a USB port -- why not just that?

Who were the early adopters of flatscreens? Airports with 486 Windows 95 or NT 3 displays -- pre-USB. Replacement screens for this sort of legacy systems are still a potential market, so there will always be a need for RS232 in some models. As most units from a particular manufacturer use the same base PCBs, RS232 support is integrated. The only option is whether to put a socket on it or not. This socket costs pennies to the manufacturer, and saves the hassle of multi-versioning their boards: retooling the assembly lines and managing parallel stocking. For this, their commercial customers can buy any model they like with minimal hassle. If it's hard to find a suitable model in your line, they'll go elsewhere. The cost of the RS232 is perfectly justifiable when you consider how many screens there are in your average airport, and how many airports there are in the world....