For certain types of artificial paving that are designed to be permeable to rainwater. This means less water runs off in heavy rainfall events, thereby reducing pressure on the storm drain system - if enough people do it - and reducing flood risk.
Gosh Rev I had no idea it could be a safety factor. Thank you for that information. I was thinking more in terms of say a pail that was porous which could not hold water or a car that's porous would get the folks riding in it wet. Porous roofs would not be good . And a porous umbrella? :)
Melonman26 