Fasting is meant to enact real change. It might seem from today's first reading that God is denouncing fasting in favor of action, but the two necessarily go hand in hand. God doesn't change, and yet Jesus says in the Gospel that "they will fast." That must mean that fasting is something desired by God after all.
But fasting isn't done for it's own sake. Like I said yesterday, the entire purpose of prayer, fasting, and almsgiving is to conform ourselves to God. Fasting is meant to be a way of denying ourselves in favor of becoming who God wants us to be. When we're not distracted by the things that give us pleasure, or medicate us into numbness, or feed our "gotta-have-it," "now, now, now" mentality, we don't have eyes to see as God sees. All we see is what we want, but God wants us to see what He sees.
Fasting is what allows us to behold the bridegroom even when it doesn't appear that He is among us. It reshapes our lens of view to see the things of God. Then we must take the next step and ask. Then we must release those bound unjustly, untye the thongs of the yoke, set free the oppressed, share our bread with the hungry, shelter the oppressed and homeless and so on.
God brings about real change when we conform ourselves to Him.