Hi John,
Yes, you are right.
Generally speaking, we hope that the temperature of the image sensor should be as low as possible during the shooting, so the sensor does not need to be heated, only need cooler. But after cooling, the protective glass will also cool, then the protective glass will dew, which will affect the shooting. So we need to heat the protective glass (not the sensor)to prevent it from dew. This is also why we call the heater as Anti-dew.
Because the dew is closely related to the environment, not only affected by the cooling temperature. So we open it defaultly.
Thanks
chad