The following is the first in a three part series about the guidelines for prayer and fasting. These are modified from Rick Shepherd's, Seeking God.
THE PURPOSE OF FASTING
Why Fast? The Scriptures clearly revel the purpose of fasting through several examples in the life of the nation of Israel and in the life of Jesus and the early church. In a survey of the Scriptures, there are at least three summary reasons given for fasting.
1) A HUMBLE ADMISSION OF NEED.
The need may be forgiveness of sin, comfort over grief/sorrow, concern over a situation, a city or a nation, or the need for clear guidance and direction from the Lord. For biblical examples, see Nehemiah 1:4-11; Daniel 9:3; 1 Samuel 7:6; 1 Kings 21:27-29; Nehemiah 9:1; Isaiah 58:5-6; Psalms 35:13; 69:10; 109:24.
2) A HUMBLE REQUEST FOR HELP.
Closely linked to the first reason, we need to admit there is a need before we ask for help. God alone is the one who can give the needed help. This may include seeking the Lord's guidance for a decision, for healing, for safety or protection. For biblical examples, see Judges 20:26; 2 Samuel 12:16-23; Ezra 8:21-23; Jonah 3:3-10. Also see, Matthew 4:2; Luke 2:37; Acts 13:1-3; 14:23.
3) A HAUGHTY ATTEMPT TO GET ONE'S OWN WAY.
This kind of fast is of no benefit as we see from the example of rebuke of a proud heart in Isaiah 58:1-4. Also in Luke 18:12, we see the ritualistic Pharisee receiving no benefit or response from God due to his fast being done for self-righteous reasons.
This should serve as a reminder that God looks at the heart not the outer man. His desire as that we would seek Him first, which at times will include fasting.