During exercising the body uses stored energy. This stored energy is known as "Glycogen".
During high intensity exercise glycogen can be depleted within 60 to 90 minutes. Blood sugar level drops as glycogen reserves are used up and lactic acid (a bi-product of energy metabolism) builds up in muscle causing fatigue and muscle cramps.
Electrolyte Sports Drinks are fomulated to help hydration and recover the body from exhaustion. a healthy body must maintain a relatively constant electrolyte to water balance.
An average body weighing 65kg contains approximately 40 litres of water - 28 litres inside cells, 12 litres outside cells and 2 to 3 litres as blood.
Any fluid that is lost during exercising or sweating must be compensated for the body to function normally. Under normal conditions the total amount of water is kept remarkably constant in a dynamic equilibrium.
But during high intensity work out in hot weather, anywhere from 1 to 2 litres of water can be lost from sweating every hour.
You can see cells keep their shape and contents at equilibrium in isotonic solutions. Water alone has not helped stressed cells.