Is Jocasta pivitol to the meaning of Oedipus Rex?
Critic and playwright Thornton Wilder said it best:
The figure of the Queen is drawn with great precision, shielding her husband form the knowledge she foresees approaching; alternately condemning and upholding the authority of the oracles as best suits the direction of the argument at the moment, and finally giving up the struggle.
Jocasta is important as a foil to Oedipus and as a trigger of his memory.
Her first role is mediator: she breaks up the quarrel between Oedipus and Creon:
Poor foolish men, what wicked din is this?/With Thebes sick to death, is it not shameful/That you should rake some private quarrel up?
Next, she dissuades Oedipus from believing in Oracles. She (and Laius) has ignored oracles since the first of them failed to come true (regarding Laius' son killing him). But, ironically, her denial is a trigger for Oedipus to question his past. So says the Enotes analysis:
...her words haunt him, he recalls "a shadowy memory,'' and asks her to give details about Laius's death.
When Oedipus begins to question witnesses in earnest, she sees the truth before Oedipus, serving as his foil. Her suicide also counterpoints his blinding. He plucks his eyes out rather than looking upon her and their incestuous bed again. She suicides because she cannot bear to live with her guilt, whereas Oedipus only blinds himself, thereby condemning himself to live knowing the truth.