The amount of water in the world is limited. The human race, and the other species which share the planet, cannot expect an infinite supply.
Water covers about two-thirds of the Earth's surface, admittedly. But most is too salty for use.
Of what is left, about 20% is in remote areas, and much of the rest arrives at the wrong time and place, as monsoons and floods.
Humans have available less than 0.08% of all the Earth's water. Yet over the next two decades our use is estimated to increase by about 40%.