The idea of allowing rainwater to fall on your roof then direct the flow by way of gutter and drainpipes into a municipal pipe network where it eventually finds its way out to sea or rivers is madness. It makes even less sense to have that same water re-diverted from a river source then processed by a utility company and returned to you when the original water is fine to use for flushing your toilet, washing you car,watering your garden and so forth.
Rainwater harvesting is the term used for the capturing of rain that fall on your property. If the water captured is filtered appropriately then it can have even more uses such as for washing machines. It can pumped either directly to the washing machine or toilet or it can be diverted to a separate storage tank in the attic.