A business might find that it needs a large pool of candidates for finding new employees. The reason might be that it has quite a few positions in order to fill or because it is looking for very specific personality attributes in order to fill some important positions. If a company is very aggressive, it might do a lot of recruiting just to fill a position that is more contentious than high-ranking. As a worker might stay with the company for decades, head hunting can be a big investment. Hire head hunting firms to handle this tedious task.
While companies might advertise positions on their own initiative, if they have a lot of recruiting to do, their own internal resources might not be up for the job. Outsource to a firm that knows how to find talent. Their ability to identify attributes that you want is top notch.