The add on kit is used to shut down the fan and damper when the boiler water temperature drops below 130 F.
This allows for a slower water temperature drop when you did not get back to add wood in time.
The minus temperature air, being blown into your furnace as it tries to rekindle a fire will cause the boiler water to drop in temp vary rapidly.
The kit and wiring instructions is for models, Polar, RG5, MD9, LG15 and Polar G Series furnaces.