Why do we often have more than (thrust reversers and brake chute) one method of slowing down (retarding) aircraft on the ground?
The multiple methods of braking help to stop an aircraft faster, both through providing different means of braking and complementing each other.
The wheel brakes are the primary method of braking on the aircraft. They work at any speed and can provide good braking force depending on the amount of friction between the tires and ground. Brakes convert the aircraft's kinetic energy into heat, and have a limited ability to dissipate that heat. Aircraft may be delayed from taking off to allow brakes to cool down after a rejected takeoff or a short turnaround. Brakes wear down over time and must be replaced. Aircraft that are light and slow may be fine with only wheel brakes, as the other methods are less effective in this case and would add complexity and weight.
Speed brakes or spoilers usually extend from the top surface of the wing. This both increases drag and reduces the lift produced by the wings, and are most effective at higher speeds. Reduced lift means more weight on the wheels, increasing their friction and the effectiveness of brakes. Airliners will typically extend the spoilers immediately after touchdown to increase braking and control effectiveness as they slow down. Aircraft are required to be able to demonstrate a rejected takeoff and stop using only brakes and spoilers on a dry runway. Techniques such as keeping the nose high as long as possible or just holding up elevator can also add aerodynamic braking.
Reverse thrust redirects some of the engine's thrust forward. This provides a braking force and for engines under the wing also helps interrupt airflow over the wing, reducing lift. Jet engines redirect core and/or fan air forward, propellers may change pitch to push the air forward. This can be particularly important on contaminated runways were low friction limits the effectiveness of the brakes. Reverse thrust is generally only used at higher speeds to reduce the risk the debris or exhaust being ingested.
These are the primary methods available on airliners. Brakes are the most versatile but require friction and must be replaced. Speed brakes and reverse thrust use aerodynamic forces at higher speeds to help slow down, and increase effectiveness of brakes. Speed brakes are basically "free", reverse thrust is relatively cheap and effective. Reducing the load placed on wheel brakes reduces temperature and wear; steel brakes deteriorate much more at higher temperatures. Having multiple methods provides some redundancy as well, but when runway friction is poor it's important that they are all used effectively to stop the aircraft.
Drag chutes are another option to slow down. Upon landing, the parachute is released and uses drag to slow an aircraft down. The parachute provides large amounts of drag but must be collected and re-packed after every landing. This is useful for aircraft with high landing speeds where cost is less of an issue, such as fighter jets or the space shuttle.
Tail hooks grab cables on the ground, and the cable provides the braking force to the aircraft. This can stop the aircraft quickly but requires an aircraft designed with a hook that can withstand the braking forces. Hooks are useful on aircraft carriers where space is at a premium, but this requires pilot skill to touch down in the right place for the hook to catch the cable. Hooks can also be used on regular runways for emergencies, where the extended hook will catch the cable as the aircraft rolls past.
I would suspect that it is due to redundance, safety and usefulness in different circumstances.
- Thrust reversers are useful to slow down from high speeds, but you wouldn't want to use them when trying to stop at a gate.
- Wheel brakes are powerful both at high speed and when parking, but can catch fire if the temperature resulting from the braking gets too high. Also less efficient on slippery runways.
- Some (primarily) military aircraft have tailhooks despite not being designed to land on aircraft carriers. The hooks provide an alternative to ejecting and loosing an otherwise good aircraft if the systems powering the brakes fail.
- Braking chutes are as far as I know mainly used on aircraft without thrust reversers; these provide an alternative to wheel brakes which is useful in certain parts of the world. Norwegian F-16 and F-35 are equipped with braking chutes as they regularly operate in areas with lots of ice and snow. Braking chutes must be jettisoned before taxiing, and has to be repacked before they can be used again, so they are less common than the other methods.
There are probably other means of slowing down I've forgotten to list. If you are on an aircraft with only one means of slowing down, what do you do when it doesn't work? Having multiple means to slow down means that any passengers or crew has a higher chance of surviving, and also that a failure can result in less damage to the aircraft.
When an airplane first lands it will be close to flying speed, the wings will still be producing lift which will keep pressure off the main wheels, making wheel brakes much less effective. Thrust reversers, speed brakes and braking chutes are designed to slow airplanes down until the wheel brakes can be used. They also reduce the landing distance needed, so runways can be shorter.