Glory to God for All Things has a very insightful post on fasting, and its purpose. You hear, in Christian circles that fasting is not a diet. Yet, in our culture it is almost impossible not to think of it that way. It is a bit like someone saying "don't think of a pink elephant". Dieting is everywhere in our culture, as is eating.
Father Stephen has a different take on the "purpose" of Christian fasting. First of all, he says, the cornerstone of fasting is inner silence and communion with God through ceaseless prayer. It is about longing for God and hungering after Him in all humility. If that is learned by a 'failed' fast, then that is a blessing.
Father Stephen had another correlation having to do with the spiritual purpose of fasting that I have never thought of. A person who is near death often refuses all food. This is a natural part of the process of letting go of this world and moving on to the next. Studies show that the dying who are allowed to forgo food experience less pain.
Fasting, in the Christian sense, then, is about dying to self and dying to the world. It is about moving on to the world for which we were created. It is about living as though we were dying because in dying, we are born to eternal life.