Humans, as well as the vast majority of life as we understand it, are composed almost entirely of water (estimates range from 60% to over 90%).
Without a constant and secure water supply, life as we understand it will not continue.
We can live for weeks without food.
But only a day or so without water.
Origins of life assumed to be in water.
The fossil record clearly indicates that life began in the sea.
Only migrated to land after several billion years of evolution.
Water at the cellular level is very important. If our cells dehydrate they die (and so do we).
In addition, water imbalances at the cellular level can occur due to exercise, dehydration, freezing, and other factors.
This can result in water/salt concentration problems which can lead to cramping, and ultimately death of the affected cells.
As weird as it sounds, if you want to lose weight drink a large amount of water.
If your cells are used to getting an ample supply of fluids, they tend to store less (so you have less "water weight").
If your cells are constantly threatened with dehydration, they will tend to stockpile additional water supplies, increasing your weight.
In addition, your body needs water to metabolize fat.
If you're dry, the fatty foods that you eat cannot be broken down.