A Watcher is an investigative role which can target another player at night and learn who else targeted that player that night. For example, a watcher watches Sophia and discovers that both Tom Tucker and KilGrey visited her. Sophia is killed during the night. Therefore the watcher knows that either Tom Tucker or KilGrey was the killer, but not which one.
Due to the fact they can confirm or condemn other people, and that there is no common "insane" version of a watcher, a watcher is one of the most powerful roles that a town can have. It can be balanced by a Ninja.
In contrast, a Watcher is a relatively weak role to give to the Mafia unless there are multiple scum teams in the game or an unusually high number of night actions in the game. Other than the slim potential to uncover multiple power roles at once, a Rolecop is a more pro-active and effective investigative role for the Mafia.