Jordan Baker helps to situate Nick in the story as a participant in the society that he is viewing and commenting on. The more involved Nick is with Jordan Baker, the more involved he is in the world she represents.
Nick opens and closes his story by distancing himself from the people and the culture of the east, but during the action of his story he is integrated into this culture. At times, he seems on the verge of fully joining Gatsby and Daisy and Jordan to enjoy the privileges of the opulently wealthy and successful.
Nick's relationship with (and engagement to) Jordan help to communicate this dynamic regarding Nick's participation in the culture of "the eggs".