On a 10 or 12 year old trailer that was probably modified by one or multiple previous owners, it's impossible to guess what they may have hooked to switches. Some things that come to mind are overhead lights, exterior "scare" lights (the big round ones), holding tank heater blankets, or possibly even controls for awning lights, under-trailer lights, backup lights or "utility lights for dump valves, etc....
The switch looks like it's not OEM simply because of the difference between those "lighted switches" and the screw type mount plate vs the non-marked, non-lighted switches in a different type mounting plate above it.
It's hard to say what they might control. As for the actual switches, if there is not a "third pigtail" on the switch to power the center contact from the hot side, the switch illumination won't work. Have you checked the back side of the switch to see if the illumination pin is even powered?
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS