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On one occasion, they accompanied Germanicus as he quelled a great mutiny among the legions on the Rhine, who were threatening the new Emperor Tiberius. They helped Germanicus to persuade the legionnaires to return to their camps and restore discipline.<ref> Tacitus, x, v </ref> For many Emperors, they were seen as the last line of defense against a mutinous army or the legions of a rival.

The Praetorians as the main military force in Rome were required to carry out a wide range of duties. Despite the sophistication of the Roman state, it had no police force or many of the other organizations that modern society takes for granted. The Imperial Body Guard played a crucial role in the policing of Rome. The city was a vast and diverse metropolis, and much of it was lawless and ruled by gangs. The Praetorian Guards were often the only force that was able to police the city. They did this was a great effect, and they helped to ensure stability in the sprawling city on the Tiber. Marcus Aurelius, for example, used cohorts of the Guards to police the city, during his many long absences on campaigns against Germanic tribes. They also appear to have been used for crowd control during the Games. Many Emperors appeared to have used the Imperial Bodyguard as a secret police force. There are accounts of guardsmen being employed to spy on real and imagined conspirators, and they appear to have been used to intimidate those who threatened the position of an Emperor.