From SA Mafia Wiki
A Lie Detector is an investigative town role who can send the Mod a single post in the game and be told if they are any lies in the post. So for instance, if a player roleclaims, that night the Lie Detector can send the roleclaim post to the mod and see if the claimer is telling the truth.
Lie Detector roles, especially if not made limited use, are very, very conducive to breaking strategies, creating situations where scum cannot even fakeclaim without being caught red-handed. Lying is the best weapon the mafia have, and taking that away leaves them powerless and helpless.
And that's terrible.