It provides a passage for air to flow out of the cabin. I cleaned mine up, but it is covered in "Jute" insulation to keep the sound down. If you've ever looked behind the bumper, below the gas cap, you'll see a little rubber flap in the body. When ever you close a door, or put the HVAC blower on high it relieves the air pressure by opening like a doggy door.
Whats odd is that there is the same body vent on the passenger side behind the jack storage box, without so much as a grill cut into the interior panel.