Fasting in Sanskrit is called upavaasa. Upa means "near" + vaasa means "to stay". Upavaasa therefore means staying near (the Lord), meaning the attainment of close mental proximity with the Lord. Then what has upavaasa to do with food?
A lot of our time and energy is spent in procuring food items, preparing, cooking, eating and digesting food. Certain food types make our minds dull and agitated. Hence on certain days man decides to save time and conserve his energy by eating either simple, light food or totally abstaining from eating so that his mind becomes alert and pure. The mind, otherwise pre-occupied by the thought of food, now entertains noble thoughts and stays with the Lord. Since it is a self-imposed form of discipline it is usually adhered to with joy
Also every system needs a break and an overhaul to work at its best. Rest and a change of diet during fasting is very good for the digestive system and the entire body.
The more you indulge the senses, the more they make their demands. Fasting helps us to cultivate control over our senses, sublimate our desires and guide our minds to be poised and at peace.
Fasting should not make us weak, irritable or create an urge to indulge later. This happens when there is no noble goal behind fasting.
The Bhagavad-Gita urges us to eat appropriately - neither too less nor too much - yukta-aahaara and to eat simple, pure and healthy food (a saatvik diet) even when not fasting.