One of the liver's primary functions is to metabolize stored fat into usable energy for the body. However, if the liver has to do some of the kidney's work, it can't operate at capacity. As a result, it metabolizes less fat, more fat remains stored in the body and weight loss stops. Overweight people require more water than thin ones. Since larger people have large metabolic loads, more water is needed for hydration and for proper kidney function.
Water also rids the body of unnecessary salt and toxins.
Water helps regulate body temperature. When in the heat, be sure to increase water intake to avoid heat stroke or heat exhaustion, or dehydration.