Over the months washing the new RX, I keep wondering what is the purpose of this plastic/rubber flap in ahead of the front tires. Anyone know what this flap is supposed to do?

 

Redirect airflow for better aerodynamics around the tires.

 

As CK6 stated, it's a "splitter" for redirection of air flow. If you loose one (which I did) the results are a potential sucking down of the inner fender liner. We lost one on a road trip and by the time we got to a Lexus dealer, the left inner fender liner had been pretty well destroyed by rubbing on the tire (bloody noisy too).