Leaves are important because Plants make food in their leaves. The leaves contain a pigment called chlorophyll which colors the leaves green. Chlorophyll makes food for the plant from carbon dioxide, water, nutrients and sunlight. This process is called photo-synthesis.
During the process of photosynthesis, plants release oxygen into the air through the leaves. The process of photosynthesis by which leaves make food for the plant is the most important process on the planet as many other plants and animals depend on plants to survive.