The Iveco ECOSTRALIS & Scania hi-box trucks have these corner deflectors that help cut side aero resistance. It is supposedly more important on crosswind aero and yaw dependance drag. The side deflectors work to diffuse the tall vertical bluff-cornered boxes.
Maybe it works like a race car side ducts, 'in front'. Wheel wells traps so much turbulent air from counter tire rotation, engine bay & brake heated air. Opposite a typical corner canard, the front vertical airflow reduce the pressure difference & air stagnation inside the bay.
ECOSRALIS Aerodynamic Kit