Those middle men are just recruitment agencies, generally. As a standard they don't have guys on staff and will take somewhere between 15%-30% on top. The 5x as much usually have full time employees and are something different, though it does happen.

For the contractor, contracting via a respected agency means you're almost guaranteed to be paid. However, they don't really offer much value beyond that. I personally dislike dealing with them but then again, my last contract didn't pay the agent, and I still got paid.