They are not needed. Even an aircraft designed to land with flaps can usually still land safely without them, albeit faster. What flaps do is enable aircraft to reconfigure the wings to allow them to land at lower speeds than they would otherwise be able to do. The shape and size of an airfoil (wing) determines how much lift (and drag) it produces in flight.
Flaps and slats change the wing's "aerodynamic shape", which enable the same wing to fly at lower speeds, for both landing and becoming airborne after take off, at a slower, but controllable speed.
Extending wing flaps for landing serves the function of lowering the stall speed of the wing, the speed at which the wing does not provide enough lift to sustain flight. This allows for a slower approach speed and facilitates the transition from flying to landing. As kinetic energy, which must be depleted on landing, is proportional to the square of velocity, small decreases in approach/touchdown speeds can make big differences in landing rollout.
The curvature of the wing is gradually increased with flaps, allowing the wing to fly slower, without stalling, increased in intervals, until it has reached a suitable speed for landing. The more flap increment, the greater the lift, the slower the speed for landing. On some airplanes, extending the flaps allows the nose to point further down and can improve forward visibility over the instrument panel or the engine/propellor.
On the other hand, retracting the flaps allows an aircraft to fly more economically, faster and with less drag when not landing. A wing in "clean" configuration can reach speeds near the speed of sound, but that would take a huge runway to either speed up, or slow down and the brakes would burn up each landing.
Flaps give additional lift at low speed, and the drag also helps to lower the speed. If the flaps are not activated, there would be much more wear on the tires and brakes, and the runway might be overshot.