What we call fasting among the Orthodox is called abstinence among Protestants. Abstinence is the abstaining from certain foods, which is what we do during our fasting periods. Fasting, for Protestants, is not only abstaining, but going the next step and diminishing the amount of food that you take in. It is what we would call a “strict fast.”
So, why do we fast? Self-discipline. Fasting is saying to our body, “You shall not have control over me.” The issue is one of self-control. It is not that the abstaining of food, by itself, is holy. The dietary laws were done away with in Christ, now all things are clean, as St. Peter is told in the Book of the Acts of the Apostles. Rather, it is that we bring all things under submission to Christ our Lord. It is the struggle to train our will so that it does not automatically respond to the urgings of the flesh, but rather to the instruction of the Holy Spirit. Fasting is to obedience to God as exercise is to the readiness of an athlete to run the race with perseverance.
An athlete puts up with weariness, muscular pains, etc., in his or her body in order to be ready to compete when that starter’s pistol finally sounds. We put up with the bother of going hungry so that when the world challenges us, we are ready to respond without fainting or running out of endurance.
But there is a spiritual side to it. As we fast, in faith, God is also there present through his Holy Spirit to help us tame our wills, to bless us, and, often, to speak to us in the deep recesses of our heart. It is “in faith” because if we simply fast as a mere obedience to rules we may not receive as strong a help as if we turn God-ward. But, let me be careful in my phrasing, if you are doing it simply to obey the rules, there is still a blessing. God does bless obedience, often by giving us the grace to stop simply obeying the rules and to begin to look at Him. He is, after all, the God of the Great Mercy.