The afternoon in the New York apartment is informative about several aspects of Nick's personality as well as the information he gleans while he is there. The first issue is why he goes there at all. The second is why he doesn't leave much sooner. Enjoyment does not seem to enter into any of the characters' attitudes toward the afternoon. Nick seems fascinated or captivated by a scene that is outside his frame of reference.
Throughout the book, Fitzgerald presents Nick as fundamentally weak and easily manipulated. He also shows Nick as harshly critical in his attitudes, although he rarely expresses those thoughts to others at the time.
Nick tells the reader this occasion was only the second time he ever got drunk. That does not explain why he stayed long enough to do so. It seems likely that he wanted to learn more about Tom's "mistress," as he earlier mentioned his curiosity about her.
Once he has had too much to drink, he feels like he wants to get out and walk, but again and again becomes "entangled in some wild, strident argument...." He is aware of the "secrecy" emanating from the apartment. He imagines himself outside, looking up at the windows, "I was within and without, simultaneously enchanted and repelled by the inexhaustible variety of life."
While the chapter is important in establishing Nick as a voyeur and outsider, it also establishes Tom's violent nature and Nick's fear of him. If Tom would hit a woman for no reason, what might he do to Nick if he did not maintain his confidence? Nick's fear and discretion are later important factors in the tragic plot.