MORE INFO:
The principle of variation is one of those immoveable tenants of training. It is the basis of the principle of overload, the premise that little progressive changes in your regime will give you better gains in performance. This goes for virtually every sport. The principle maintains that a constant adaptation and changes in intensity, volume and time of the workout will prevent the stagnation of performance gains or increase performance gains faster than a repetition of the same workout. Thus by overloading the body in different ways, you constantly force the adaptations and therefore increase sports performance.
SOURCE: