Rainwater Harvesting is the simple process or technology used to conserve rainwater by collecting,  storing, conveying and purifying of rainwater that runs off from rooftops, parks,roads, open grounds,etc. For later use. 

Recharge of rainwater to the ground prevents sea -water immersion in to the fresh water bodies in the coastal  areas.

Rainwater  Harvesting can help in maintaining the under ground water level .