[aerohead]

Without the spat,the air is attacking the forward face of the tire at road velocity,while the forward face of the tire is descending vertically downwards at the same velocity.
Evidently,the chaos created by such a situation is worth some intervention.
With CFD,or a tunnel,the automaker can 'tune' a low-cost piece of thermo-formed plastic to divert the air in such a way as to optimize the interaction for the lowest drag,short of enclosing everything,which if done,would destroy the styling.