We need water for several reasons. Below are some of them:
A larger percent of the human body weight is water! And this water is lost or leaves the body all the time. The major channel through which water is lost from the body is through sweat and urine. Lost fluid from the body needs to be replaced regularly if you must stay healthy. Two ways in which we get this water is through food and drinks.
We cannot survive for more than a few days without water. In fact we die fast without any form of water especially when the weather is warm. We can live for a longer period without food but not without water.
Dehydration will definitely set in when you do not drink enough water.
We need water to regulate the body temperature, thus, preventing it from over heating or under heating.
We need water to help flush out waste from the body.