Question: If flaps don’t increase lift, then why is a small amount of flaps used in soft-field takeoffs for some aircraft?
Answer: Who said flaps don’t increase lift? Flaps do increase lift. Reference Chapter 6 of the Pilot’s Handbook of Aeronautical Knowledge, which states, “Flaps are the most common high-lift devices used on aircraft. These surfaces, which are attached to the trailing edge of the wing, increase both lift and induced drag for any given angle of attack (AOA).”
When flaps are deployed, they can lower the stall speed, which is why some aircraft manufacturers recommend the use of a minimal flap setting during a soft-field takeoff. The aircraft is able to lift off at a lower airspeed under positive control, and when the aircraft is off the ground and has enough flying speed the pilot retracts the flaps and continues the climb out at the appropriate V-speed.
Some aircraft do not require the use of flaps for takeoff unless it is a speciality takeoff—like a soft-field departure—and others require some flaps to be used on every takeoff; the amount is spelled out in the pilot’s operating handbook (POH) or aircraft flight manual (AFM).
Too much flap and the drag diminishes the usefulness of the lift—for example, if the POH calls for 10 degrees and the pilot mistakenly inputs 20 degrees, that airplane is going to eat up a lot of runway on the takeoff roll and the pilot may be fighting the elevator to get the airplane into the air—if liftoff happens at all. Before adding power, verify the required flap setting and the aircraft configuration, per the POH.