Basically, designers face a certain problem with airplanes. At high altitude, during cruise flight, you want to fly as fast as possible while when landing you want to fly as slow as possible. Therefore, at high altitudes you need less wing area than at low altitude flight and the net result is that at low altitudes, the wing area is extended by the flaps and slats etc.
They do not slow the aircraft down, or at least that is not the main intention. That is done by the air brakes. Flaps generate lift and therefore they ALLOW the pilot to fly slower.
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