For some fascinting biblical examples of the purpose of fasting:
The Disciple's Fast:
"To loose the bands of wickedness" from ourselves and others. Isa 58:6, Matt 17:21
The Ezra Fast:
"Undo the heavy burdens." Isa 58:6, Ezra 8:23
The Samuel Fast:
"To let the oppressed (physically and spiritually) go free." Isa 58:6, 1Sam 7:6
The Elijah Fast:
"To break every yoke" conquering mental and emotional problems. Isa 58:6, 1Kings 19:4, 8
The Widow's Fast:
"To share our bread with the hungry" and to care for the poor. Isa 58:7, 1Kings 17:16
The Paul Fast:
To allow God's "light to break forth like the morning," bringing clearer perspective and insight as we make crucial decisions.
Isa 58:8 Acts 9:9
The Daniel Fast:
So "thine health shall spring forth." Isa 58:8, Dan 1:8
The Esther Fast:
That "the glory of the Lord" will protect us from the evil one. Isa 58;8, Esther 4:16, 5:2