1. It creates a nostalgic, meditative tone as Didion reflects on how quickly her time in New York seemed to pass.
The images of the revolving door and the fountain water turning to snow are ways of showing the time passing.
2. It helps Didion convey the idea that her initial conception of New York City was based on intangible ideals and fictional depictions rather than concrete experiences and reality.
In the passage she says "us" which eliminates the last option. She talks about how she is the one who had idealized notions of the city. The sentence following this excerpt talks about how she had a "romantic notion" of the city. This supports that her view of the city was based on tangible ideals and fictional depictions.
3. She mentions that they all had a plane schedule in the drawer, ready to fly back to where they came from on short notice.
In the text she talks about how they were on "extended leave from wherever they belong" and that they have plane tickets in their drawers.
