1. Fasting increases our sense of humility and dependence on the Lord (for our hunger and physical weakness continually remind us how we are not really strong in ourselves but need the Lord). 2. Fasting allows us to give more attention to prayer (for we are not spending time on eating). 3. [Fasting] is a continual reminder that, just as we sacrifice some personal comfort to the Lord by not eating, so we must continually sacrifice all of ourselves to Him. 4. Fasting is a good exercise in self-discipline, for as we refrain from eating food, which we would ordinarily desire, it also strengthens our ability to refrain from sin, to which we might otherwise be tempted to yield. 5. Fasting also heightens spiritual and mental alertness and a sense of God’s presence as we focus less on the material things of this world (such as food) and as the energies of our body are freed from digesting and processing food. 6. Fasting expresses earnestness and urgency in our prayers: if we continued to fast, eventually we would die. Therefore, in a symbolic way, fasting says to God that we are prepared to lay down our lives that the situation be changed rather than that it continue.
Wayne Grudem