Rainwater harvesting provides an independent water supply during regional water restrictions, and in developed countries, is often used to supplement the main supply.

It provides water when a drought occurs, can help mitigate flooding of low-lying areas, and reduces demand on wells which may enable groundwater levels to be sustained. It also helps in the availability of potable water, as rainwater is substantially free of salinity and other salts.

Application of rainwater harvesting in urban water system provides a substantial benefit for both water supply and wastewater subsystems by reducing the need for clean water in water distribution system, less generated storm-water in sewer system, and a reduction in storm-water runoff polluting freshwater bodies.

Source: https://en.wikipedia.org/wiki/Water_resources

Source: WHO/Unicef Joint Monitoring Programme