Tom keeps an apartment in Manhattan for his meetings with Myrtle Wilson. She uses it as a home away from home. She is able to meet Tom there at practically any time because she has a sister named Catherine who lives in Manhattan and she can always tell her husband she is going to visit her sister. The fact that Tom is providing an apartment for Myrtle is significant because it shows that he is having a long-term affair with her and therefore must not be having much of a marital relationship with Daisy. This is one of the reasons that Daisy is considering getting a divorce and marrying Gatsby. Daisy is well aware that her husband has a mistress, and this seems to excuse her in her own eyes for engaging in an adulterous affair with Gatsby. The infidelity of Tom helps to sway the reader’s sympathy towards Gatsby in spite of the fact that he is very aggressively trying to break up Daisy’s marriage and jeopardize her relationship with her three-year-old daughter. Tom would probably have the legal right to obtain custody of the child if he decided to do so, and there is no indication that Gatsby would want to become a stepfather to Tom’s daughter.
The appartment that tom has in New York is so he has a place for his mistress, Myrtle Wilson. Although both himself and Mrs. Wilson are married.
Tom has an appartment in NY so that he has a place to take Myrtle Wilson, his mistress