As your body is composed of 70% water, it is important to remember that any sort of physical exertion results in the occurrence of sweat. When you exert yourself, you can emit up to 2 liters of sweat or one percent of your body weight. Hydration avoids the negative side effects such as overextension in your heart rate, cramping, extra fatigue and increased body temperature. Keeping your heart rate low helps you to maintain a high level of intensity while exercising. It is important to hydrate as you exercise, but also before hand to give your body an ample level of fuel comparable to putting fuel in your gas tank. Hydrating initially and throughout provides the best chance for success.
While this is fairly common sense, what people often do not realize is that they also sweat out sodium as well. Refilling on exclusively water dilutes the amount of sodium in your kidneys forcing them to work harder at about roughly 0.7 grams per hour. Thus most hydration drinks (think Gatorade or Powerade provide enough sodium to make sure that they will operate at the right level. The moral of this story is just to tell you to bring water on your next adventure to avoid dehydration.