When you think about it, Yoda in the movies is pretty much the archetypal GM plot-revealing mouthpiece character.
This sort of character is required even more in roleplaying games, where you can't expect the players to know the entire backstory of the campaign world, including every little legend and myth that might become important to the plot later on. Even though the characters might logically be aware of some of this stuff, the players surely won't be. Unless they've actually read and absorbed the multi-page "campaign history" document you handed out at the beginning of the campaign, and remembered the salient facts.
Yeah, we don't think so either. Enter: the Yoda.
Obi-Wan: Master Yoda, I have failed. How can I go on without my master?
Yoda: A master no longer do you need. Confer on you, the level of Jedi Knight the Council does.
Obi-Wan: Even after I couldn't save Qui-Gon?
Yoda: More Jedi knights, we need. Of the Sith, know you?
Obi-Wan: Uh, no.
Yoda: A legend, there is.
Yoda: A warlike race they were. The Force for evil, they used. Destroyed themselves, they did. But a dark legacy they left: ones who to their ideals would dedicate themselves.
Obi-Wan: But if this is just—
Yoda: Dark forces at work, are! Resurfacing, the Sith may be. It, the Council senses. This boy, the Chosen One he may be. To avoid bringing Balance to the Force and fulfilling the prophecy, train him carefully you must.
Obi-Wan: I think I better take some levels in understanding garbled prophecies first...