A dirty or clogged cabin air filter can cause musty odors in the vehicle
and cause contaminants to become so concentrated in the cabin that
passengers actually breathe in more fumes and particles when riding in
the car compared to walking down the street. A restricted cabin air
filter can also impair airflow in the HVAC system, possibly causing
interior heating and cooling problems, important for staying comfortable
this winter. Over time, the heater and air conditioner may also become
damaged by corrosion.
Source: Air Filter