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They were developed for brake cooling efficiency, not (primarily) for making the airflow at the outer side of the wheels less turbulent. This does not mean, however, that aero efficiency is not the goal:
They absolutely must keep the brakes cool over a 2-hour race distance with hundreds of multiple G-force braking events, with the carbon discs getting up to about 1500 deg surface temp. If the brakes overheat the race is done. The way they used to ensure adequate cooling was to just increase the brake duct size, which increases drag, but also, more importantly (for racing), contaminates the airflow over vehicle surfaces, esp. the rear wing, which reduces downforce. These wheel covers with strategically placed vents, in conjunction with inner ducting and disc covers, allow the same amount of cooling from smaller brake ducts. The wheel covers certainly must aid the extraction of the brake cooling air in part by reducing turbulence over the tire/wheel face, but the reducing turbulence is not their end purpose. In short, the design is not for making the brakes cooler, but for improving aero efficiency (mainly downforce).
As to how the wheel covers do not rotate with the wheels, you can see the pit men rotate the wheel cover when they first put the wheel on until it locks into place. I've never heard it explained other than by reference to a ball-and-detent mechanism.
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