People have been collecting rainwater for a long time to save drinking water and due to its beneficial properties. The only difference today is that the rainwater barrels are safer and more efficient, while also made of much more durable materials.
If you plan to purchase a new water barrel, find out if it is child proof, protected against leaves and insects, equipped with special devices which will route the excess rainwater from the barrel to canalisation or other place near your house. You can use the collected rainwater to water plants and wash your car or house floor.
You can also use water from surrounding water bodies to water plants - you can gather it in buckets or use a pump and a hose to water your garden directly.