Once you're past the nits, waiting to be picked; it gets interesting.
www.wrcwings.tech/2018/09/19/wheels-impact-on-the-aerodynamics-of-a-wrc-car/
It would appear that spreading the interaction of the oncoming air up the face of the tire reduces the interaction, and substitutes for a spat.
It's interesting to see how WRC cars are dialed-back time attack designs. The wheel diffuser would be less vulnerable to debris. Anyone with an old pickup truck with running boards could take note.