While the human brain is often thought of as the “control center” for all of the body’s processes, there is really only one system that directly uses the brain. That is the central nervous system. The brain and the spinal cord make up the central nervous system together, and send signals throughout the body, fueling your body’s sense, movements, reactions, and so on.
The brain also controls processing, integrating, and coordinating any information that the body receives through the sense organs, then guides decision-making processes and sends signals to the rest of the body on how to respond to the information it has received. So while the brain only works with one system in the body, it is still one of the most important organs in the human body.