Fasting is a very personal matter, usually done with a specific concern in mind. Therefore, the purpose of the fast and the special considerations of the individual involved govern the motivation and the nature of the fast.
Most of us fast in conjunction with our membership in the Church and its law of the fast. Generally speaking, there are three purposes for such a fast. First is to increase humility and spirituality of the individual fasting. Second is to provide assistance to the needy by contributing fast offerings equivalent to the value of the food which has not been consumed. Third, physical benefits may be derived personally.
In the General Handbook of Instructions (1968, p. 40), we read, “A proper fast day observance consists of abstaining from food and drink for two consecutive meals, attending the fast and testimony meeting, and making a generous offering to the bishop for the care of those in need.” Thus, the use of water is excluded in this kind of a fast.
Now, a word of caution—some fallaciously reason that if a little of anything is good, a lot is better. The inadvisability of excessive fasting was explained in some detail in the June 1972 Priesthood Bulletin, “We are informed that some … engage in rather lengthy fasting. It is not advisable that they do this. If there is a special matter for which they should fast, if they would fast one day and then go to the Lord humbly and ask for his blessings, that should suffice.” Moreover, Joseph F. Smith wisely counseled, “Many are subject to weakness, others are delicate in health, and others have nursing babies; of such it should not be required to fast. Neither should parents compel their little children to fast.” (Gospel Doctrine, p. 244.)
The generous offering to the bishop is understood to represent the financial equivalent of at least two meals. A liberal donation so reserved and dedicated to the poor is ennobling to the soul and helps one develop charity, one of the greatest attributes of a noble human character. (See 1 Cor. 13.)
The personal benefits derived from fasting are substantial. The scriptures tell us that a certain kind of devil goes not out except by fasting and prayer. (See Matt. 17:21.) The supremacy of the spirit over the appetites of the body is affirmed by the mental discipline of fasting. This strength fortifies us in our combat with other temptations prompted by physical appetites that, if uncontrolled, would be destructive to our welfare. While some have physical conditions that preclude fasting, most people are not excluded on this basis. To me, a successfully completed period of fasting from food and drink on fast day brings a degree of self-confidence. Fasting is real evidence to oneself and to his maker of gratitude for the gift of health and strength which permits one to be able to fast. Surely this is a great privilege and blessing.
-Russell M. Nelson, "I Have a Question," Ensign, Apr. 1976.