This is common misconception probably coming from level flight situation. Commercial aircraft wings are made for adequate lift in takeoff and minimal drag in level flight. Which encourages building wings that generate all necessary lift in level flight while being relatively "flush".

Generally, when flaps and slats are extended, they increase the camber of the airfoil, increasing the Coefficient of Lift for the same AoA. Also, the air gaps introduced at the junctions between slats/flaps and the main wing re-energize the boundary layer on the top surface of the wing to retard flow separation on the upper surface (becomes a concern because of the increased camber).

They actually do both, depending on their deployment.