So when you are heating or defrosting a car, you have two choices. Outside air, or Recirculate.

Recirculate means you are keeping the air in the cabin instead of taking in new air and pushing the old air outside. Don’t worry, you still get some air exchange. On an EV, recirc is important so you aren’t throwing overboard air you spent battery to heat. On a gas car, recirc is less important since heat is free.

However, recirc has a problem: your breath and perspiration and wet clothing are adding water to the air, and when you recirc, the humidity gets higher and higher. Hot air can hold more water than cold air, so when it hits the cold windows, they fog or frost.

The cure for that is to run the air conditioner (i.e. normal A/C mode). The cooling element is before the heating element in the heating ducts, so this acts as a dehumidifier, chilling the air, removing humidity, then re-heating the air. Obviously, running the A/C when it’s below freezing outside is “pumping downhill”, so it’s rather efficient. It also provides a source for the heating you do next. That’s why some cars have separate A/C evaporators and heat pump condensers in the ducting.

When water evaporates, it cools you. That’s what sweating and wind chill are all about. When water condenses, you get heat. You gain about 300 watt-hours for every pound of water you condense. And this water evaporating in your lungs, skin and clothes was chilling you, making you want cabin heat, so the dehumidification of the cabin earns that heat back. That’s why it’s nice to recirc and dehumidify instead of tossing that moist air overboard as you might do in an ICE.