Water is lost from the body at very high rates.
All together, the body loses almost 11 cups of water a day. The kidney loses the most, followed by the skin and lungs. Since the body itself makes some water, and since we get between 2 and 4 cups of water from food, this is why 6 - 8 cups of water a day is recommended.
Replacing water losses from the body is very important. Failure to replace water loses from prolonged sweating, can lead to heat exhaustion.
If you tend to eat a high protein diet, it is important that you get adequate fluid to help eliminate the waste products.
Drinking the right amount of water can also help you avoid constipation problems. During periods of illness, when you are experiencing vomiting, diarrhea, or high fevers, it is necessary to replace those fluids to prevent dehydration.