There's a lot going on in a small area there. Even without helicopters, the main runway (01/19) is the busiest runway in the nation, and it points directly at a no-fly zone over the white house, so the approach has a complicated turn at the last moment. Directly across the river, there's a military base with a heliport. And those helicopters often transport important individuals inside of those areas and to areas up and down the river. Those helicopters aren't just casually flying through, they are doing things in the immediate area.
Just as an example, look at a map and take note of where DCA is, where the Marine One hangar is, and where the White House is. All of this stuff is right around the airport.
Just as an example, look at a map and take note of where DCA is, where the Marine One hangar is, and where the White House is. All of this stuff is right around the airport.