Harvesting rainwater doesn't really do anything good for the environment because whether or not you collected that rainwater it would eventually just end up in the ground where it would naturally be stored anyway.
But if you can catch the natural rainwater coming from the sky you do not have to extract that water from a water well or other place.
The water would still be available years later but by collecting rain water you can use it for flushing your toilet, washing your car, watering your garden and much more.
You eventually save a bit of money on your water bill by not having to by that water from your water company or city.
All that rainwater would just get soaked into the ground anyway and end up in the cities water supply so you keep it out of their supply and use it without having to pay.
The rainwater is very natural and does not contain any chemicals so it makes the natural rain water great for gardens.