From Richard Foster's Book Celebration of Disciplines - regarding Fasting:
The Purpose of Fasting
Fasting must center on God. It should draw us closer to God, where we will experience a deeper relationship with Him. Like Anna, in Luke 2:37, we should be “worshiped night and day, fasting and praying.”
Fasting reveals the things that control us. We tend to cover up what is inside us with food and other good things, but in fasting these things surface. If pride controls us, it will be revealed almost immediately. David said,“I put on sackcloth and humbled myself with fasting.” (Psalm 35:13) Anger, bitterness, jealousy, strife, fear - if they are within us, they will surface during fasting. At first we will rationalize that our anger, for example, is due to our hunger. We will then discover that we are angry not because of hunger, but because the spirit of anger is within us. We can rejoice in this knowledge because we know that healing is available through the power of Christ.
Fasting helps us keep our balance in life. How easily we begin to allow nonessentials to take precedence in our lives. How quickly we crave things we do not need until we are enslaved by them. Paul wrote, “Everything is permissible for me"--but I will not be mastered by anything.” (I Cor. 6:12) “No, I beat my body and make it my slave so that after I have preached to others, I myself will not be disqualified for the prize.” (I Cor. 9:27) David wrote, “My knees give way from fasting; my body is thin and gaunt.” (Psalm 109:24) That is not asceticism; it is a discipline - and discipline brings freedom.