Leaves are important because they are the primary source of photosynthesis, which is how plants feed themselves. Photosynthesis is the process of turning light energy into sugars, which plants need to survive, so leaves are very important to a plant's overall health and survival.
If plants can be seen as metaphorical cities, leaves are the farm or food factories of the plant. Green leaves contain chlorophyll, which is not only how the leaves get their green color but also a major ingredient in the photosynthesis process. In addition to energy from light, plants need water and carbon dioxide to make sugars.