The Brainliest Answer!
1. To make water available where there is a scarcity.
2. make water availabe where the supply is irregular.
3. To conserve the surface run-off during monsoons.
4. To recharge the aquifers and increase availability of ground water. 
5. To improve the quality of ground water where required.
6. To overcome the problem of flooding and stagnation of water during the monsoon season.
7. To arrest salt-water intrusion.
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The main use of Rainwater Harvesting is to Conserve the water for our FUTURE use.