Conflicts in the workplace
Many people are nervous, anxious, afraid of bringing up conflicts in the workplace, believing they will not be believed, or they will be dismissed or not empathised with and helped. They might trust HR, they might not and the odds are more towards the latter. That's not an insult, that's just the reality in conflict situations, especially if there is a power imbalance between the people.
Bringing in a professional mediator or if necessary, having one on staff, separate from HR, is much more likely to lessen doubt and anxiety and improve confidence to pursue problem solving earlier and effectively manage or resolve conflicts before stronger, significant or severe damage is done, leading to the need for a crisis situation response at a considerably higher expense.