It is always a good idea to have stored water on hand for irrigation or in case of emergency. The best part - rain water is free! 

Water harvesting had its beginnings in pre-history throughout the world. Many civilizations relied heavily on the capture and storage of rain water for survival. 

A word of caution, though, just because you are using rain water does not mean it is pure and good for drinking. Water Purification is extremely important if you will be drinking your harvested water. Also, some areas have laws regarding collecting rain water.