What does Air Vent's internal weather filter do?

It's secondary level of weather protection.

Air Vent's internal weather filter provides a more complete barrier from the elements. Wind-driven rain, snow, dust and insects are trapped by the filter. And because Air Vent ridge vents provide continuous airflow along the entire roof, the internal weather filter is cleaned by the flow of air coming out of the attic. The ridge vent's external wind baffle is the primary weather protector.