Humans, animals and plants on Earth need the sun because it provides most of the energy required to survive in this solar system. Earth would be a very cold and lifeless planet if it did not get its light and heat from the sun. Similarly, the Earth's stays in orbit because of the gravitational pull of the sun.
Plants also need the sun's energy for the photosynthesis process used to manufacture food and oxygen. While a plant is an autotroph that make its own nutrients, animals and humans are heterotrophs that depend on autotrophs or other heterotrophs for food. Without food and oxygen animals and humans could not live on Earth.
Humans also need sunlight to manufacture vitamin D. This vitamin is necessary for the absorption of calcium that is important for strong bones and overall health.