Here you can see it's multiple uses
Turning vanes, the vertical plates, usually curved, mounted on either side of the chassis between the front wheels and side-pod intake first appeared on cars in the early 1990's. In the late 1990's and early 2000's they began to appear more complex, often more than one element on the car, and were put in more places such as under the nose-cone and at the edges of the side-pods. The 2009 regulation change made the turning vanes simpler and restricted their positioning, further implemented by the 2019 effort to make overtaking easier.
Turning vanes, are called so because they turn airflow around the car to reduce drag from the exposed side-pods. They have many functions, but the primary one is to control the turbulent wake from the front wheels and to grab some air from the front wing and the under chassis area. This turns the airflow to the side-pod, which is good for cooling, floor and diffuser (which are good for downforce).
Recently, engineers have put complex vortex generators that direct vortices under the car, with the aim of feeding the rear diffuser. This increases energy flow to the diffuser and therefore downforce too.
Also, bargeboards are to guide turbulent air from the front wing wake away from the vital underbody (this is a separate element from the one that guides the air to the diffuser). In addition, the lower edge provides vortices along the side of the car, acting as a side skirt, stopping other, potentially turbulent, from getting underneath the car (lift risk).
A modern complication
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