Every millimetre of rain on one square metre of roof is one litre of water. Delhi NCR's ~790 mm year means a 100 sqm roof sheds nearly 79,000 litres - most of it in a few monsoon weeks.
Storage systems filter roof water into a tank for gardening, washing and (with treatment) more. Recharge systems send filtered water underground through a pit or a defunct borewell, raising the water table you draw from. Recharge is cheaper and maintenance-light; storage gives you usable water on tap.
Many NCR municipal bodies mandate RWH above certain plot sizes and some offer property-tax rebates for compliant systems - worth checking with your local body, and a good dealer will know the current rules.