F1 Wheel Cover
There is a brake duct on the inside of the assembly which channels air over and around the disc and caliper. It appears the vent in the wheel cover allows airflow from the interior of the wheel to exit more efficiently.
This enables greater temperature reduction by allowing more air to flow through the wheel and brake than with a rotating cover. On a rotating cover a vent or opening would probably only be effective for a portion of each revolution of the wheel, thus requiring more openings and inducing more aerodynamic drag.