I have read that they are pressure relief vents. If the air inside the car can't get out, the outside air can't get in through the vents/heater & A/C systems. They function as a "calibrated air leak" to ensure continuous airflow through the car.

 

You are on to something here is an article and video that describes how they also prevent back pressure when you close the rear hatch and doors. There are one-way flaps between the vents and the outside:

Here's What Those Secret Rubber Flaps Behind Your Rear Bumper Are For