Purpose

Fasting should always be done for a purpose. 

"Without a purpose, fasting can be a miserable, self-centered experience."

Here are the 10 major catagories - main reasons to fast:

1.  To Strengthen Prayer.  There is something about fasting that sharpens our prayers, that gives us passion, and urgency.
2.  To Seek God's Guidance.  Have a big decision to make?  There is a Biblical precedent to fast while praying about the choice.  It is said to help give clarity and make us more receptive to God's love and guidance.
3.  To Express Grief.  Grief for a death, for our sins, for the sins of others, or the sins of our nation.
4.  To Seek Deliverance or Protection.  Protection from enemies, from abuse, from gossip, from those who seek to put down and criticize our faith - all those things that threaten to make us retreat from the world.  We can fast and pray that God be with us in those dark hours.
5.  To Express Repentance and a Return to God.  We can express our grief over our sins - and then make the choice to change our ways.  Fasting can be a signal of that new commitment to God.
6.  To Humble Oneself Before God.  Fasting, is a physcial expression of humilty before God.  (Which is another reminder not to brag about it to others!) 
7.  To Express Concern for the Work of God.  A person might fast and pray for the work of God in a place that has experienced a tragedy, disappointment, or apparent defeat.  Some feel burdened to fast for the cares of the world - and seeing how they may be of service.
8.  To Minister to the Needs of Others.  This is an unselfish fasting - concern for others brings you to prayer and fasting.  An example, fasting a meal once a week and using that time to volunteer or giving the money to a charity.
9.  To Overcome temptation and Dedicate Yourself to God.  Think of Jesus in the wilderness - facing temptations and when overcoming them, dedicating himself fully to God.  Sometimes we face tremendous temptations in our lives, fasting is one way to seek and ask God for guidance and strength in those moments. 
10.  To Express Love and Worship to God.  An act of sheer devotion - an act of finding your greatest pleasure and enjoyment from a life with God!

--Donald Whitney, "Spiritual Disciplines for the Christian Life"