Can determine the sanity of a role. Roles without alternate sanities appear sane (debating whether Fool, Killer, Amnesiac, etc. should appear insane). Sided with the village.
Think about this: the gunned person is outted on lylo but the GS's sanity is unknown. If the MD thinks the gunned is mafia he can claim that the GS was sane while really he is not. A mafia would then take the shot for the easy win while town would continue to think about their decision. This is one example of a reaction test and a good way this role can be a leader.
Psychologist, Psychiatrist, and Therapist wouldn't work. With more roles with alternate sanities being added I think this role becomes necessary, especially for ones like gunsmith who can't tell their sanity until it's too late.
crazybruce