They give legislators information about subjects the legislators need to know something about in order to do their jobs.
I mean, right at the beginning, after we see Kevin Spacey (superb) talking to himself in a mirror, we hear his explanation of why lobbyists exist. I had a difficult time dealing with this movie, partly because the entire system of lobbying is so despicable in itself, and partly because the writer has done his best to show Jack Abramoff as a fundamentally nice guy who just overreached a little and got caught.
