Why are administrative agencies sometimes referred to as the "fourth branch of government," and is this an accurate representation?

Because legislative,judicial,and executive powers have traditionally been placed in separate branches of government by the United States Constitution,the role of administrative agencies has led some to refer to agencies as the unofficial "fourth branch" of government.Administrative agencies are not actually a branch of government,primarily because all of their authority is merely delegated to them.Despite their delegated authority,administrative agencies remain under the control of the three traditional branches of government.