Think of water as the U.P.S. system of the body. Water can better lubricate your joints. If you are sitting all day long without a great deal of movement, and you are getting stiff, it's a sign to consume more water. Think of all those fluids that surround your joints. The fluid is mostly water and it needs replenishing.
Body temperature has a lot to do with water.
If you are hot or running a temperature, then it's a good idea to consume even more water. If you're trying to lose weight, then drinking water is even more critical. It helps your digestive system, and gives your stomach a fuller feeling. Obviously, coffee and colas contain a lot of water. They also, in most cases, act as a diuretic, which means you'll need to drink even more water.