The cabin air filter is designed to filter the outside air that enters the cabin of the automobile.   The outside air enters through vents usually at the base of the front windshield.   The air conditioning component shown above is the evaporator out of an older vehicle.   This older vehicle was not equipped with a cabin air filter.   As you can see a build up of debris has collected on this unit restricting the transfer of cold air to the cabin.   This debris could consist of bird feathers, weeds, dust, dirt, molds, and more.   The smell of the air passing through this unit may not be pleasant.   The heater control unit on the dash gives you an option to recycle the air that is in the cabin.   By pushing the recycle button to the on position you close a door that shuts off the outside air.