The professional recruiter acts as a bridge between parties on either side of the hiring process. Not only do they have a highly developed understanding of the client's needs and how the candidate can serve them, they've determined whether or not the company and position will serve the candidate.
Negotiation carries with it the connotation that there is always a loser—that only the strong survive. Contrary to that idea, the professional works to create symbiosis. Both parties exist in a mutually beneficial relationship—a win win. This state of affairs must be achieved, or the job is not finished.