IIRC,
Flaps momentarily increase lift, before its increased drag ultimately drags down the speed so it causes more harm to lift.
They are also a secondary flight control which use is almost mandatory in take-off/landing procedures in RL. They are stabilization devices that lowers the stall speed of the plane. If a plane stalls at 90mph with flaps up, it might hold on until 70~80mph if flaps are out.
So, while it is not advised for combat use in RL, and rarely did normal pilots ever use flaps in combat, in harsh maneuvering conditions it could prove to be very useful.
For instance, if a P-51 is chasing a Bf109 at high speeds, the 109 pilot would be hard pressed to enter a tight turn due to physical limitations such as dangers of tunnel vision, fatigue, and heavy elevator controls. The chasing P-51 could drop a notch of flaps and it would;
1) act as a speed-brake which slows down the aircraft, so its speed comes closer to the corner velocity, helping it turn better and avoid overshoots
2) provide more lift, due to the flap effects, and thus help the turn
3) stabilize the plane, due to its nature as a stabilization device, which also helps in a turn
Often, a plane that maneuvers 'worse' in a certain point of view, might outmaneuver a 'better' plane solely due to flap effects. Take the example of 109 vs P-47s.. if there's a good P-47 in the cockpit, the 109 will have a very hard time trying to follow prolonged, tight turns, since the stabilization effect of the P-47 flaps have a very benevolent effect on the plane, as compared to 109s in AH which cannot start using its own flaps until under 190mph or so.
Or, the example of Ki-84s which can stay with a Spit5 once its flaps start popping out.
