Water is vital to your body both as a solvent and as an essential part of many metabolic processes within the body. It helps digest your food, transport waste, and control your body temperature. Without water, these metabolic processes could not exist and your body would begin to malfunction at a cellular level.

In short, water is essential to human life.

And while you are outside, it is easy to get dehydrated. Interestingly, dehydration symptoms become noticeable after 2% of your normal water volume has been lost http://www.skillsforwildlives.com/2012/01/dehydration-its-importance-for-the-wilderness-traveller/