1) A small amount actually increases the flavour of food.
2) It increases the temperature at which water boils, speeding up the cooking
3) When boiling vegetables with no salt in the water, the natural mineral salts within the softened cells of the vegetables migrate into the water and are lost. This is not good - your body needs these. Adding salt to the water creates a mineral balance between the water and the vegetable cells, and the minearal salts stay within the vegetables. This is better for you. But with too much salt in the water, the salt migrates into the vegetable cells, and the food tastes salty. Avoid this.