Flaps do all the things that people have said so far, but there is one thing no one has said. Flaps not only increase lift at slower speeds, they also increase glide slope angle. You can use flaps to make shorter landing approaches at slower speeds than without flaps and this is true for any airplane, model or full scale. Flaps should only be used at low speed. Full size airplanes have a maximum flap usage speed printed right on the airpseed indicator and while modelers don't have that luxury, don't use flaps at high speeds.