The ultimate purpose of the Jedi Order can, I suggest, be discerned from the Jedi Code, somewhat akin to the Athanasian Creed, a five-line koan which represents their own (self-perceived) highest ideals:
There is no emotion, there is peace.
There is no ignorance, there is knowledge.
There is no passion, there is serenity.
There is no chaos, there is harmony.
There is no death, there is the Force.
This indicates that their highest ideals revolve around self-possession, scholarship, calm, harmony etc, hence all the talk about seeking balance in the Force, which first means seeking balance within oneself (which is why <PERSON> kept counselling <PERSON> to take control of his own wild emotions at first instance, when he kept worrying about his wife, the Council, the galaxy etc). It is similar to Taoist and Buddhist ideas of balancing the various forces within oneself - not a balance between Sith and Jedi or the Light and Dark sides of the Force (as some have suggested), since the Dark side would by definition be the result of an imbalance. Hence the Order's reclusiveness, austerity and emphasis on meditation and "sensing" things.
The overall goal of all this is greater and greater unity with the Force (cf. the saying "be one with the Force"), i.e. a Buddhist or Hindu-like quest for mystical union, which would finally result in transcendence - total absorption into the Force, as the last line indicates. 503
(The Jedi also appear to believe that this merging with the Force also occurs at a Jedi's death.)
So where does this leave the rest of the universe? There is a famous Sanskrit saying, om mani padme hum, which likely influenced <PERSON> (given that he named a major character <PERSON>!). This saying has many meanings, but basically asserts that the macrocosmos is an external reflection of the internal soul. In this context, it would mean that seeking balance within the external world is part of seeking balance within oneself, so as part of a Jedi's mission of transcendence, they would feel obliged to also bring balance, serenity, harmony, etc to those around them (filling the function of the Buddhist bodhisattvas). Hence, "guardians of peace and justice throughout the Galaxy". (Now whether or not they were any good at actually achieving this is another matter!) 42