Undertaking investigations into grievance and disciplinary matters is incredibly demanding of management time and attention, and take expertise and focus away from other core strategic functions.
Without consistency in how investigations are carried out, and in the
quality of the work done, you are more vulnerable to appeals, and people who are tasked with investigating a complaint but who do not have proper training for the role may open the employer up to challenges at Employment Tribunals.
So, many employers chose to use external investigators to work on individual complaints, because there are several benefits to doing so.
This paper explains what these are.