We can't see anything without light. It is a form of energy. Plants use this energy to prepare food by utilizing other factors necessary.
We often feel that the water we pour to the plants is the source of food for them. But water is one of the factors of photosynthesis.
The nutrient chemicals in the water are transformed to carbohydrates with the help of chlorophyll in the leaves. The water from the land is sucked (absorbed) and transported to all parts of the plant through the stem, by the roots.
Leaves absorb CO2 from the environment, with the help of sunlight; they split CO2 to carbon and oxygen. They combine nutrients with carbon to prepare carbohydrates.
Even though a plant has green leaves, nutrients, CO2 it can't prepare food in the absence of any other factor also (like CO2, chlorophyll, water).
It is estimated that approximately 150-200 tons of carbon element is used by the leaves every year during photosynthesis.
Whatever may be the food item (rice, pulses, vegetables, fruits, nuts etc.,) we use in food (which come from plants) is prepared by photosynthesis.
Thus leaves are considered as food factories of the plant.