Most life on Earth depends on photosynthesis, the process captures energy from sunlight to produce oxygen. As the primary sites for photosynthesis, leaves manufacture food for plants which in turn nourish and sustain us all. Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.
Leaves provide us with so much, even after they fall they continue to feed the earth and the animals who live within the soil. Withouts plants, simply there is little life.