The main point of intermittent fasting is keeping your insulin levels low for the longest time possible, increasing insulin sensitivity, and forcing the body to use stored fat for energy.
Reduced calorie intake is the bit that makes you lose weight. Increased insulin sensitivity is a result of weight loss. "forcing the body to use stored fat for energy" is the mechanism by which weight loss happens.
Having separate debit and credit amounts (instead of a single positive/negative number) serves another purpose as well: To track the total amount added or deducted from each account.
The main point of intermittent fasting is keeping your insulin levels low for the longest time possible, increasing insulin sensitivity, and forcing the body to use stored fat for energy.