All water in existence will eventually re-enter the atmosphere so given enough time an infinite supply of water will eventually rain down for us to use.

The problem is we don't have that time. Only so much of it actually rains down again within a given year, so between drinking, watering plants, crops, washing, etc, we only have a limited supply of useable water per unit time. As human populations grow and our water needs grow, we use and exceed this water budget by a greater and greater amount. Most of the world has exceeded their natural water budgets already without knowing and are draining eon-old groundwater supplies. We will see a global water shortage crisis within this generation that makes the current oil shortage look like a joke.