Aside from the positive effects of meditation found by psychological studies (improved memory, relaxation, lowered anxiety, better sleep, etc.) here is a reasoning that provides a more immediate (and infinite) reason for this ancient practice:

When we are in mental and physical motion our consciousness is bound by the limitations of the mental and physical reality it functions in. Though these processes are experienced internally and externally, they both still reflect the limitations of the perceived external reality. Only when the mind and body are stilled and quieted may our attention turn to focus inward more deeply than in our immediate understanding or feelings of things. Here our consciousness may flourish without the boundaries of physical or mental constructs. By gazing into the self with a fearless observation and an honest, non-judgmental acceptance we may see right through the self to transcend the ego. Here the consciousness may rest at peace with all that is or drift as freely as the ever-changing wind in a timeless, limitless existence.