Recruitment specialists typically have databases that help to store and track candidate information and applications and network with candidates as well. They also have established relationships with a wide and diverse network of professionals and job seekers from which they can use to find your company a great fit for a role.
Recruiters external to your organization are also better able to act as a liaison between your company and competition companies to seek for currently employed candidates who may be interested in a change of job.
It is worth using their services if you do not have time to locate a candidate, or the hire is a particularly critical role to the success of your organization.