A garden hose or a lawn sprinkler can flow at around 4 gallons per minute, while a good low flush toilet uses around 1 gallon per flush. This means that just one hour of watering could use 240 gallons of water, which would be equivalent to 240 toilet flushes or a dozen 10-minute showers with an efficient (less than 2gpm) shower head.
Even drip irrigation systems can use a lot of water if run frequently and/or for extended periods of time.
The main point being. The water necessary for human health and sanitation is only a fraction of the water that is often used for lawns, gardens and shrubs that are not drought tolerant.
David Slade