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.)
In England the water companies routinely dump raw sewage into waterways, to the point where it isn’t safe to swim or even go near the water in many places and there are massively deleterious effects on ecosystems. It’s a huge scandal at the moment because the formerly public and now privatised water companies are doing this dumping while continuing to pay huge dividends to shareholders and the toothless regulator is doing nothing about it.
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.)