Perhaps they were brakes only as a by-product of their original intent?

"Early in 1940 the 109 airframe was redesigned for maximum aerodynamic streamling intended to take full advantage of future improvements to the DB 601 engine. With the underwing radiators causing a significant portion of the excessive drag, a boudary layer bypass system was developed, which collected turbulent air on the underside of the wing, directed it over the radiators, and ejected it from a duct on the upper surface of the lower flaps, allowing the upper and lower flaps to act in opposite directions to serve as a radiator flap."