What is the importance of the zipline in the novel, Allegiant?

Several times before, and in previous novels, Tris and some of the other Dauntless have zip-lined out over the city in Chicago. Tobias has always turned Tris down whenever she's asked him to try it with her. She tells him that the zipline makes her feel alive, free, and in control of her own destiny to make choices. At the end, Tobias, to honor her, spreads her ashes as he makes the descent via the zip line. It also represents his coming to terms with her death by this act.