In Shakespeare's Twelfth Night, how is Feste significant in the main plot?

In Shakespeare's Twelfth Night, Feste is the fool in Countess Olivia's household. He has served the house for years, entertaining Olivia's father until his death, but Olivia criticizes him over his frequent absence; however, he continues to spend the play entering and exiting the action seemingly at random. His primary job is to entertain, and he does so with songs, dances, wordplay, and mockery of the people around him. Feste's role as a character is primarily pointing out the flaws and absurdities in the behavior of the other characters in the play. For example, he mocks Olivia for her self-indulgent mourning, and points out the excessive moodiness of Duke Orsino's behavior. Feste's presence in the play seems almost omniscient, as he seems to know more about the characters than the audience does and appears in the midst of important actions or conversations frequently. His role in the play is to inform the audience about the characters, make moral judgements about their behavior, and mock the absurdity of their actions. He exists outside of the society presented in the play, which puts him in the position of judging and mocking the inner workings of that society.