Okay, before I used to go on for running 2 hours continuously without stopping to drink water and hydrate myself. I thought it was just fine. But recently I've known its importance. It's important because proper hydration of the body keeps the body functioning properly and just aids in the proper metabolism. Also dehydration causes uric acid build up. When one works out, he uses up a lot of calories, as a byproduct of this, he sweats. The body needs water to metabolize and the sweat also comes from the fluid provided by the water in the body. So water is used up more during work out so dehydration is very likely to occur. When one lacks water in the body, his cells will shrink and of course, he's gonna feel weak. During a work out, feeling weak will decrease the workout efficiency. Instead of performing better, one would be performing worse.