When there is snow on the ground, moisture gets on the inside of the car, and it evaporates when the car warms up. When the car sits for a while and it is cold outside, frost forms on the inside, and/or on the outside.
I could just see the zigzag pattern of the wires embedded in the glass, so the direct heating defroster was doing something. But the areas where the air defroster hit the frost melted quickly, while the area away from the air flow stayed frosty for quite a while.
I will see how it goes in other situations that require the use of the defroster. I want the direct heating to work, because it is (supposed to be) much more efficient, and it has a minimal effect on the range.