It's also a revered part of "the formula", mainly for aesthetics. I'm guessing they want the cars to look more like the other formula series.
Yeah, add wheel covers and you basically have the LeMans prototypes instead.
To keep them cool, because grip is everything and you don't want them too hot or cold. Sweet spot is key.
In this kind of series the tyre’s overheating and degradation characteristics are carefully designed in to them. The factory has taken in to account covered/uncovered airflow on the wheel.
You control overheating by tuning camber and caster.