Stereo & Electronics - whats the vent for found in drivers side wheel well

In starting my Dynomat installation in the rear hatch, I've removed the carpet in the wheel well area. In taking out the extra padding and such found onthe drivers side, I see this rectangular, black plastic vent of some sort.

It has 2 rows that are covered but can flap open. The placement would be right behind the rear red turn marker on the drivers side.

Whats this thing for??? There is not one on the passangers side.

its for cabin pressure. this is so you can close your doors easily without having to force against the air in the cabin and blowing your ears out.