The position of Prime Minister is usually one of high power in any government. It is usually the head of government and is in charge of most government departments and ministries, hence it being the main minister, or Prime Minister.
In most cases, the Prime Minister is not the leader of a country or state but rather the leader of its government. The duty of a head of state is usually left to another figure, possibly of ceremonial importance.
Prime Ministers of the Empire had considerable authority and influence but were mainly political leaders and did not intervene much in military or religious affairs, prefering to leave that to designated officials.