The most important reason to develop concentration is as a base for stronger, deeper insight. In bringing the mind to tranquility, concentration wears away the forces of greed and aversion that move the mind and keep us agitated. When the mind is under the sway of likes and dislikes, wanting and not wanting, it doesn't see clearly. When those are stilled, the mind sees more clearly. Concentration also brings a tremendous sense of wellbeing and ease. We access it through relaxation, and in its wake comes peace and stability.