The soil harbors millions of organisms per cubic inch. Not only bacteria, but fungi, larger microbes like amoebas, and multicellular critters--like earthworms, insects, millipedes, centipedes, mites, spiders, and all kinds of creepy-crawlies. No doubt you're familiar with these larger inhabitants of the soil--the ones you can see.
But in the size range smaller than visible, yet much bigger than the microbes, is a whole world of animals. Legions of wiggling things, rooting between the tiny grains of soil. All these, from the worms to the microbes, are eating, breaking down, and metabolizing the pollutants that sink into your rain garden. If one critter won't eat that oil from the street, or that antifreeze, the next one will.
At great cost to taxpayers, the sewage plant tries to recreate what's going on in the rain garden, in order to digest your sewage. But your rain garden does it without a pipe, does it locally, and for no cost.
In a rain storm, when pollutants wash off our streets and down the storm sewer, they head straight to the lakes. Once there, bacteria and algae try to break them down. But it's harder to do in the lakes. There isn't as much oxygen, and there aren't as many critters as in your rain garden . So the lakes start to stink, and who wants to swim where pollutants are being digested? Better to let your rain garden do it.
It used to be that most of our water pollution came from big pipes--sewage plants or factories discharging into our lakes and streams. Once we cleaned those pipes up, it came as a surprise that our waterways were still going downhill. The culprit turned out to be "nonpoint source pollution." This is shorthand for pollution from a myriad of tiny sources-- your dog... your auto... your lawn. Most of these tiny sources are spilling onto pavement, and rain gardens can trap and purify those pollutants.
There are a myriad of sources of pollution, so we need a myriad of rain gardens. Sure, you can pick up after your dog, and use less fertilizer. All excellent ideas. But the rain garden is the final trap for the nasty little things that slip by our defenses.
All those ravenous little monsters in your rain garden are just waiting for their next meal.