Last Sunday night we studied Jesus' words in the Sermon on the Mount regarding fasting. One of the things that stands out is that fasting is nowhere commanded in Scripture, but rather is simply expected of those who follow the Lord. In preparation for the lesson, I found these paragraphs from Calvin's Institutes to be of great help, especially in answering the questions: "Why should I fast?" and "What good does it do?"
15. A holy and lawful fast has three ends in view. We use it either to mortify and subdue the flesh, that it may not wanton, or to prepare the better for prayer and holy meditation; or to give evidence of humbling ourselves before God, when we would confess our guilt before him. The first end is not very often regarded in public fasting, because all have not the same bodily constitution, nor the same state of health, and hence it is more applicable to private fasting. The second end is common to both, for this preparation for prayer is requisite for the whole Church, as well as for each individual member. The same thing may be said of the third. For it sometimes happens that God smites a nation with war or pestilence, or some kind of calamity. In this common chastisement it behooves the whole people to plead guilty, and confess their guilt. Should the hand of the Lord strike any one in private, then the same thing is to be done by himself alone, or by his family. The thing, indeed, is properly a feeling of the mind. But when the mind is effected as it ought, it cannot but give vent to itself in external manifestation, especially when it tends to the common edification, that all, by openly confessing their sin, may render praise to the divine justice, and by their example mutually encourage each other.
16. Hence fasting, as it is a sign of humiliation, has a more frequent use in public than among private individuals, although as we have said, it is common to both. In regard, then, to the discipline of which we now treat, whenever supplication is to be made to God on any important occasion, it is befitting to appoint a period for fasting and prayer. Thus when the Christians of Antioch laid hands on Barnabas and Paul, that they might the better recommend their ministry, which was of so great importance, they joined fasting and prayer (Acts 13:3). Thus these two apostles afterwards, when they appointed ministers to churches, were wont to use prayer and fasting (Acts 14:23). In general, the only object which they had in fasting was to render themselves more alert and disencumbered for prayer. We certainly experience that after a full meal the mind does not so rise toward God as to be borne along by an earnest and fervent longing for prayer, and perseverance in prayer. In this sense is to be understood the saying of Luke concerning Anna, that she “served God with fastings and prayers, night and day” (Luke 2:37). For he does not place the worship of God in fasting, but intimates that in this way the holy woman trained herself to assiduity in prayer. Such was the fast of Nehemiah, when with more intense zeal he prayed to God for the deliverance of his people (Neh. 1:4). For this reason Paul says, that married believers do well to abstain for a season (1 Cor. 7:5), that they may have greater freedom for prayer and fasting, when by joining prayer to fasting, by way of help, he reminds us it is of no importance in itself, save in so far as it refers to this end. Again, when in the same place he enjoins spouses to render due benevolence to each other, it is clear that he is not referring to daily prayer, but prayers which require more than ordinary attention.
Jean Calvin, Institutes of the Christian Religion, IV, xii, 15.