A bunch of cars these days are air-tight enough they experience "flutter" where you feel an uncomfortable vibration like blowing into a big jug but much, much lower and stronger. To battle the "flutter" all it takes is something to disturb the airflow.
The little bitty plastic piece does that easily.
When you have the top down at high speeds, stick your hand up in the airflow and "feel" it's direction. Very different in the center!