Focusing only on internal options could result in the company overlooking a highly qualified candidate who would have been a better fit for the role. You could limit your options which could cause you to miss out on solid candidates. It could also lead to a lack of fresh perspectives and new ideas and approaches. There could be a potential for bias as well if there is not a clear and fair process.
External recruiters can offer an expanded talent pool, including passive job seekers that are not on job boards. Using an agency recruiter can introduce new approaches and ideas to hiring and can open up more options for candidates. Agency recruiters who specialize in your niche have the industry know-how and contacts. They can also make suggestions on your recruiting and hiring process since they have seen many other recruiting and hiring processes across other client’s organizations.
An external search can promote objectivity.