Conducting the process in-house typically means distribution of an ad to your organisation’s network, and posting it to a few job boards. Then you sift through applications, in a bid to shortlist and interview. Important to note here, is that when you advertise online, you are only reaching 15%  of potential candidates;
those who are actively looking for opportunities. Passive candidates not only make up a much larger proportion of the population, but they are on average higher quality candidates. These passive candidates are not actively looking for work and therefore, not trawling SEEK and traditional job boards. Good head-hunters and recruiters nurture diverse and relevant talent pools of passive candidates and can often grant you access to them, deepening your talent pool of prospective candidates. So unless your network is broad, you may not be casting your net wide enough.