Does Praetorian Guard play a role on battlefield?

[Frank]

Not saying they weren't a bodyguard, of course - but they had other functions too. Ten thousand men is rather excessive just to guard a single man, after all... even if, as Tacitus implies, only one cohort at a time was on duty at the palace.

Thats what I thought, too. The initial intention of Augustus was most propably to supress even the idea of a civil revolt by the aristocrats or the plebs urbana. But just to be present becomes a boring job pretty soon. Bored praetorians are a waste of manpower and a risk, too. So I am convinced that their daily tasks were more than just guarding a few public buildings and the periphery of the emperor. Unfortunately there are not that many hints about their additional daily tasks in ancient sources.