In Pittsburgh, as with many older US cities, we have a combined storm water / sewer system. Stormwater runoff from impervious surfaces joins the sewer system; under normal conditions, this means we have to pay to clean it, and under high flow conditions, it means the sewers go into the overflow system, dumping feces into the river.

This isn't a universal problem, but it means there's a pretty big benefit to reducing runoff, and the city has been pursuing it aggressively for the last 20 years or so, to good effect.

(The runoff also leads to localized road flooding, which is again both dangerous to road traffic and expensive to mitigate.)