In my case? My plane sits outdoors in Alaska. At 15*F there isn’t much going on inside the engine. What moisture is in there will be represented as frost on the surfaces. If I plug in and heat the engine it converts that frost to water vapor and provides a convective environment where the vapor rises to the top, meets cool surfaces, condenses into liquid water, drips down, converts back to water vapor, and repeats the process. With vulnerable assemblies like cam, lifters, cylinders, etc living in the greenhouse. I prefer frost.