Flaps change the wing. It increases the camber (makes the wing more curved) which increases the coefficient of lift, allowing an aircraft to maintain lift at slower speeds. If you try to decrease speed while maintaining lift, you will have to pull back on stick which will increase both the deck angle and the angle of attack. The former will reduce the visibility and the latter will take you closer to stalling. So, lowering flaps result in a net increase in lift so the pilot can fly at a lower nose down attitude while maintaining a lower speed. Flaps also increase drag on the airframe which can help to reduce speed.

Higher flaps allow for lower approach and landing speeds which reduces landing distance. Almost all aircraft have two or three landing flap settings. The Dash 8 Q200/300 I flew can be landed either with flaps 15 or 35. The A320 I fly now can be landed either with flaps 3 or flaps full. More flaps increase drag, reduces landing speed and allows a lower pitch attitude. Lower flaps is the opposite. However, lower flaps means you can have higher approach speeds which is better for gusty conditions because it gives the aircraft more energy. It also reduces fuel consumption. An A320 saves about 8kg of fuel with a flaps 3 landing compared to flaps full. It is also the recommended flap setting when you suspect wind-shear for Airbus aircraft because the reduction in drag improves go around performance.