The way they were explained to me by an engineer that does commercial and residential development is: If you put buildings on and pave 5 acres of open ground that once absorbed a good portion of a 1" rain and now almost 100% of that 1" rain now runs off immediately. So to control flooding that runoff is contained by the storm sewer system and released over a longer period of time. It still ends up in the storm drain system just not all at once. I don't know about filtration or reduction of solids. This may differ geographically.