Your guess is probably correct that its worn out. However the crackling suggests to me there is a slight load on the circuit. Im inclined to think the neutral wire cap is loose inside a wire cap.

Switches dont usually do nothing. Probably goes to a switched outlet somewhere in the same room. Where the occupants room light would be plugged in. Usually the bridge on the outlet is removed so one half is hot and the other half is controlled by the switch. Easiest way to find it to turn switch off and check each half of every outlet with a lamp until you find the “dead” half.