We fast so we can use our body's natural reflexes to converse wth God.  We struggle with Jesus, turning pangs of hunger and our burps into intimate prayer, a deeper accompaniment with Jesus. 

Fasting is a marvelous and irreplaceable form of prayer; it's the prayer of the senses.

"One form of prayer rests another."

We fast for a bit of self mastery as well, letting our will and our intellect regain a bit more control over the body and the appetites in little and subtle ways.

Very valuable, helping us grow a bit inside, so that later on when we're tires or bothered and when we need to love our family..by changing a diaper, or going out into the rain to pick up milk, our body or our "self" won't rebel and complain as much and we will be more able and prepared to do our duty immediately and cheerfully for our wife, making her life a wee bit more pleasant.

So fasting in the right way disposes us to many acts of charity.