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The only thing I can think off is their compressor was not working and they wanted to have it turn on and off by itself without it freezing. You see that thermostat cuts signal once it gets to a freezing point, so possibly connected the thermostat in between the compressor and sent power to it. If they line freezes the Tstat would cut the signal.
Not a bad way to rig it however I question the efficacy of it being that that line varying in temp since it's so close to the exhaust. That Tstat moved up by the brake reservoir would have worked better.
If your compressor is working along with everything else, remove it.