People need water because water transports nutrients to the cells of the body, adds moisture to the internal organs and tissues and helps to flush toxins out of the body. Water also eases digestion by preventing constipation and helps to keep the body temperature stable.
Water also cushions and lubricates the joints, allowing them to move easily and painlessly. It also keeps the eyes and skin moist and supple. Water allows the lungs to inflate and deflate. It is especially important to drink water during illness, when doing intense exercise and during periods of hot weather.
Water is also important for cleanliness. Water washes away bacteria and other pathogens to protect the body from disease. It's also necessary to clean instruments and buildings. This protects them from dirt that might corrode the material they are made of. Water is also necessary in cooking and can be used to put out most types of fires.
Water is used extensively in construction and manufacturing. It's also crucial for in science and medicine. Water used in irrigation makes it possible for humans to live in arid places where they normally couldn't live. Dams use the power of water to produce electricity for entire cities.