The picture at righ shows air flowing over the front of a bicycle wheel with a small aero section rim. This means you use more watts, a box type rim is worse and a deeper section rim helps straighten these jumbled air currents out. The photo on the left shows air flowing off the back of the same wheel. Airflow off the trailing edge is where real speed comes from on aero wheels. The rim shape dictates how attached the air will be as it flows off the rear and into the frame tubes.
Smooth air uses less energy to pedal thru.