Streets, basic sanitation, garbage pickup, police services, etc.
That may not add up to a billion, because there are a lot of "parasitic" stuff added on, but they account for a good chunk.
My point. A billion is a lot of garbage pickup. However, salaries at $164k for city workers indicate that someone is getting a good ride out or the taxpayers. Atlanta is even worse - has about twice the number of city workers as DC for about the same size city.