Its simple, its how our bodies work and function!
If you sleep 8 hours a night, guess what? You were fasting...
If your body requires 2,000 calories of energy to operate at optimal levels, it will either get those calories from food or stored body fat. (Your body is ALWAYS eating, even when your NOT.)
In a 24 hour period if you consume 2,000 calories and thats within 12 hours, then take 12 hours to burn off those calories (Fasting) which will give you balance.
If you eat 2,000 calories in a 4 hour period and allow your body to fast for 20 hours. Your body will burn off the 2,000 calories and then access your stored body fat to burn as fuel instead of food.
Fasting is simply the time frame your not eating to allow your body to burn stored body fat, heal and repair your entire body.