A/C is used to Dry the inside Air in the winter.
If you have ever had a cold beverage on a warm day you may have noticed the cold glass gets wet on the outside, that's not a leaky glass, lol.
Condensation occurs when warm air comes in contact with a cold surface(glass) or cold air, like clouds forming.
That is what happens inside a vehicle when it is cold out, warm air inside comes in contact with cold window glass and water vapor forms on glass, fogs ups.
Because cooling the air causes this condensation all A/C units have a drip pan, as the air is cooled moisture is removed and drips into the pan, on vehicles there is a drain hose so this water can drip out.
To help unfog vehicle windows it is best to use dry air, so if vehicle has A/C it us used to dry the air more before blowing it across the windows.
Usually the A/C will only come on when Defrost is selected, but it can be used when any heat is selected as this drying of the air will prevent fog from forming on any windows.