Which sin does the Tzitz atone for?
Rashbam: It atones for any sin on which one brings an Olah, a Chatas or an Asham to atone - together with the respective Korban.
Targum Yonasan: It atones for Chutzpah. 3
Not for Kohanim who brought Korbanos be'Tum'ah (See Rashi).
And not for the sin of Pigul or Nosar (See Rashi).
Which explains why it is worn on the Kohen Gadol's forehead ('Metzach' - since Chutzpah is also known as 'Azus Metzach'). By the same token, the Gemara in Erchin (16a) points out that each of the Kohen Gadol's Begadim atoned for a specific sin ... The tunic for murder, The pants for adultery, The hat (turban) for arrogance, The belt for immoral thoughts, The Chyoshen Mishpat for pervesion of justice, The Eifod for Avodah-Zarah, The Me'il for Lason-ha'Ra, The Tzitz for Chutzpah. See Gemara for the sources.
Bearing in mind that the Kohen Gadol only wore the Tzitz whilst performing the Avodah, why does the Torah insert the word "Tamid"?