[Def]

To make front downforce with a production car, you are trying to get as much air to flow under the splitter as possible.

The rear diffuser helps this by helping "pump" the air from the front to the rear. Same with small diffusers in front of the front wheels.

The venting of the rear of the front fender is trying to act somewhat like a rear diffuser - pumping more air out, which increases the mass flow of the air under the splitter, which drops its static pressure (Bernoulli's equation).