A storm sewer to carry off storm runoff
What if the street you live on had only a curb built around it, with no stormwater intake such as the one pictured here. Any low points in your street would collect water when it rained. And if your street was surrounded by houses with yards sloping uphill, then all the runoff from those yards and driveways would collect in a lake at the bottom of the street. That might make it fun for you to splash around in, but the guy at the bottom of the street would need a boat dock instead of a garage. It doesn't take much rain to cause problems. In fact, I've seen local flooding even on streets that have storm sewers.
Here's an intake designed to gather runoff from the yards and houses bordering a street. These intakes are put at low points on the street and collect storm water and send it on its way to local creeks through large pipes buried underground.
By the way, these intakes are also known to eat large quantities of kids' toys and balls that are left out in the rain.