In image dark grey wind guide directs wind downwards (red line) and causes tyre contact angle to be much over 90 degrees.
Without wind guide tyre contact angle would be worst 90 degrees. It goes directly opposite towards tyre midline.
Compared to typical pre tyre spats which are wind "curtain" style because air can not normally pass upside are causing vortexes. This is not the best option. Basicly problem is exactly same as a front of car.