The metaphor of the aerialist: What does it tell us? : Let the Great World Spin

Yes, as anticipated, more the strands in Colum McCann's "Let the Great World Spin" are coming together as the novel nears completion (I've almost finished reading "book" 3 of 4). I'd thought McCann had "forgotten" one of his characters, the mother (Claire) whose son has died in Vietnam, but we pick her strand up again through the POV of her husband, whom we'd last seen on his way to criminal court (he's a judge); he will preside in the afternoon session and first hear the case of the two prostitutes from the Bronx, Tillie Henderson and her daughter, Jazz; then he will hear the case of the aerialist (never named in the novel, though obviously based on Petit). So the two major story lines converge in the courtroom. Among other character developments: in the section on Tillie in prison (she seems to be preparing herself for suicide, distressed that she cannot visit with her grandchildren), we see her meet with the artist who caused the car accident (that killed Corrigan and Jazz), and she is now having an affair with Ciaran Corrigan (Corrigan's brother, first character introduced). We see Corrigan and his Guatemalan girlfriend as they fall in love, on the day that he will die. We also begin to learn about the Bronx neighbor who has apparently adopted Jazz's two children, learning about her impoverished life in Missouri. In short: a tremendous amount of material, all very well developed, built on excellent research and observation (especially the courtroom chapter) yet the research never overwhelms the story. It's much easier to follow this plot than I can convey in these snippets of plot summary. My only concern about the novel: does the metaphor of the aerialist really carry the weight of this book? What's the guiding metaphor about? How or why is it necessary? What does it tell us about the city then - and today, now that the towers have been obliterated?