If you run low on water, your body can overheat, like a car that is low on cooling fluid. Losing just two percent of the body's water can hurt performance. A five percent loss can cause heat exhaustion. A seven percent to ten percent loss can result in heat stroke and death. Dehydration can kill.
Young athletes have a lot of growing to do.
New muscle tissue must be made. Bones need to grow rapidly. And with all of the physical activity, some tissues need to be repaired. All of this metabolic activity requires an abundance of nutrients and energy carried to body tissues and waste products carried away. Water allows all of this to happen.