I have 1981 house in Washington state with in-wall electric heaters and mechanical single pole thermostats. I want to replace thermostats with something less crappy. I unscrewed one of the thermostats and wiring I saw confused me: there were 3 cables (3 wires each: black, white and bare copper) wired like this (BL, WT and BC are black, white and bare copper wires respectively, grouped by cable, SW1 is a thermostat):
I was expecting to see two cables there: one from electric box and another to the heating unit. Why 3? The only guess I currently have is that the room had two heating units installed initially and then one was removed. No idea why wires are still connected though. Or am I missing something?