What is the primary function of the EMER EXT LT switch when set to ARM?

The primary function of the EMER EXT LT switch when set to ARM is to activate emergency exit lights. When this switch is in the ARM position, it ensures that the emergency exit lights will automatically illuminate in the event of an emergency or loss of power. This is critical for passenger safety, as it helps guide occupants to exits during an evacuation. The emergency lighting system is a vital feature of aircraft safety, providing a clear path for passengers to follow when visibility is compromised.

The other options do not relate to the core function of the EMER EXT LT switch: arming security systems, powering off all lights, or charging external power batteries are functions associated with different systems within the aircraft, thus they are not applicable in the context of this switch's purpose.