These prayers were not peculiar to Good Friday in the early ages (they were said on Spy Wednesday as late as the eighth century); their retention here, it is thought, was inspired by the idea that the Church should pray for all classes of men on the day that Christ died for all. Duchesne is of opinion that the “Oremus” now said in every Mass before the Offertory—which is not a prayer—remains to show where this old series of prayers was once said in all Masses.
— Catholic Encyclopedia (1909)