I have collaborated with my colleague, Dan Binstock (managing director of the BCG office in DC) to put together an article on the benefits of working with a search firm. In rereading the article, I realize that there is one potential benefit that we forgot to mention.
If you are a lawyer with great credentials but you have something in your background that needs explaining (e.g. a gap in employment, an involuntary termination), a recruiter can help you get over this hurdle by being your agent. In other words, the recruiter can explain the problem to potential employers in a more neutral way.