to allow the heated air to escape during headlight use. Occassionally, some very humid air will come in through the vent naturally or by a high-pressure wash. This can sometimes result in some condensation. Turning on the headlights should heat up the air, push the humid air out through the vent, and dry up the light. If you have a chronic problem with condensation, you probably have a bad seal, as this should be a rare and temporary occurence.