What is the purpose of the cabin air filter?

Cabin air filters remove dust and pollen from the outside air entering the AC system.

Cabin air filters remove dust and pollen from the outside air entering the AC system.Re-circulated air should already be cleaned by the cabin air filter as it entered the car.

All sources and heated air only are ruled out by the filter being in the outside source, which supplies air for ventilation, heating, and AC operation.