An in-house H/R person may have maps to the city, but lack the familiarity to keep you out of the bad part of town. The recruiter on the other hand, knows it like the back of their hand. Not only would they keep you from danger, they know all the construction projects, detours and dead ends that frustrate the journey from Point A to Point B.
The professional regards open dialogue with the client on any given issue a duty. This is in the client's best interest, but something an internal recruiter may be disinclined to do. Vulnerability to internal politics and cultural dimensions hinders the insider—the outsider is less affected by such things.
Professional recruiters have a clearer view of the position than the client themselves do. The professional can help the client clarify all elements of the position—scope, expectations, salary requirements, personality type sought and so forth.
Too many hiring authorities fail to understand that a professional recruiter's primary function is not to fill a slot but to provide the right candidate to solve a problem. They clearly define the position and how it should contribute, then find the perfect fit.