Fasting is the deliberate, temporary abstention from food or some types of food for spiritual reasons. Sometimes one can abstain from both food and water.
It obviously becomes starvation when it is not done with prayer, meditation God's Word, Worship...
Here we go,
1. To receive instructions from God. (Acts 13:2)
2. To appropriately serve God. (Luke 2:37)
3. It is in the will of God for us to fast. (Matt. 6:16-18) Jesus taught about it.
4. To discipline our soul. (Psa. 69:10)
5. To draw closer to God and oppose evil spiritual forces (Joel 2:12; Jugd. 20:26-27)
6. Gain revelation knowledge of God's will. (Dan. 9:3, 21-22)
7. Help during severe trials. (Acts 27)
BONUS: Try to fast at least once a week, so that you may grow and mature in Spirit. Fasting doesn't kill.