Hi Thao!  Thanks for your question!  Good headhunters (HHs) should be able to give you unique insights into the industry and into many clients they serve.  They should be able to give you a very good idea about industry trends, what skills and competencies clients value, and specific examples on how people within their clients have manged to move up the career ladder.  Getting the latest information on compesation and benefits is also helpful!  A good HH has direct and often times not readily available information on companies, industry trends, or salary information that can be useful for you.  Lastly, and perhaps more important, a good HH can connect you with people within their network, which can enrich your own network along the way.