Cuts down on the backdraft created by the air trying to fill the low pressure area behind the wind screen as speeds increase.  The backdraft becomes more pronounced the taller the windshield.  It also helps exhaust fumes from being sucked back into the cockpit especially with rear exhaust and a roof. 

Lots of convertible sports cars use them. 

The only rear window deflector with tight tolerances and bottom brackets for stability.  No whistling or rattling.  Available in all colors to match the Slingshade 4 piece window kit.