I would imagine it works on same principle as an aircraft engine inlet duct , which on fighters etc (and on some f1 cars- barge boards etc) is positioned slightly away from the main body, as it removes the turbulent boundry layer of air by effectivly "peeling" it away from the smooth airflow. turbulent air is inherent in any intake system,by keeping the airflow laminar into the compressor as much as possible it is far easier to conrol surge/stall etc