For over a thousand generations, the Jedi Knights were the guardians of peace and justice in the Old Republic.
That was Obi-wan explaining who the Jedi were in A New Hope.
It stands to reason that there are threats to peace and justice other than Sith. Before Episode II, the Jedi hadn't encountered the Sith in a thousand years, and yet still stood as "the guardians of peace and justice".
Furthermore, the prequels are instructive, because we get to see how the Jedi operate and are viewed before their downfall. I don't have quotes at the moment but you are given the impression that the Jedi are 1) frequently deployed as uncommonly effective emissaries and negotiators by the Republic to deal with situations like illegal embargoes, 2) serve as protectors of threatened senators, and will chase down and investigate assassins, across the galaxy if need be, and 3) serve as generals during wars, though it's impossible to say how common this was, if at all, before the Clone Wars.