Why do we often have more than one method of slowing down (retarding) aircraft on the ground?
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.