The cabin air filter is one of those parts people often remember only when the car starts to smell bad or the blower feels weak. In reality, the filter collects dust, pollen, soot and other fine debris every day.
When the filter becomes clogged, the ventilation system has to work harder and airflow gets worse. That can mean windows fog up more easily and the cabin no longer cools or heats as well as before.
A clean cabin filter matters especially for people with allergies and for anyone driving a lot in the city. A dirty filter can noticeably reduce air quality in the place where you spend hours every week.
A practical rule is to replace the cabin filter at least once a year or according to the manufacturer interval. If you drive on dusty roads or in dense urban traffic, a shorter interval may make sense.