 Unlike grass and dirt, asphalt and concrete are impervious, which means that water can't get through it. Rather than soak into the ground, water runs off streets into storm drains, which is how it gets to the sewer. Not only does that mean more work for the sewer system, but stormwater can pick up all kinds of nasty stuff on the roads, from trash to chemicals to bacteria. And if water can make it to the ground instead, it can filter through the dirt instead of running along the road.