Why does Fitzgerald add the detail about the dog biscuit to the description of the party at Myrtle's apartment?
The dog biscuit is to a 'begging' dog as is the party in Myrtle's apartment. Fitzgerald is making an analogy between the lure of a dog biscuit to a dog is not all that different from the lure Myrtle has for her apartment in NYC. Both the biscuit and the apartment are provided by those who have the power and means to provide. In essence the dog and Myrtle are one in the same. Both can be given 'treats' however both are subject to the whims of their 'masters'. Remember, Fitzgerald is writing his novel with commentary. He questions the materialistic nature of society, and how better to get the point across than by comparing the materialism of a human being to that of a begging dog.
In F. Scott Fitzgerald's novel "The Great Gatsby," Fitzgerald uses the dog biscuits that the elevator boy picked up (on his own initiative after reluctantly going to fetch a box full of straw and some milk) as a means to explain two things. Firstly, the elevator boy was reluctant, but went to "fetch" the items for Myrtle, because he was merely an elevator boy. Someone with money, like Tom (and at least for the time that she was with him, Myrtle) could easily have some lowly elevator boy go fetch them what they wanted. Having money means being able to give orders to people who are their to "serve" the rich (or at least that's the way Fitzgerald shows how people with money think and view others in lowly service positions. Obviously, the elevator boy wasn't going to object and say no. Yet later, when it is just Nick and Mr. McKee and they are leaving, after Tom breaks Myrtle's nose, the elevator boy snaps at Mr. McKee, telling him to keep his hands off the elevator lever! -Which symbolizes the elevator boy's disrespect for someone who doesn't have money, especially after he was just associating with people who behave in the way that both Tom and Myrtle behave - bossy, rude, and "better than people" to whom they feel they can bark orders at. The elevator boy was establishing that these two weren't going to bark orders at him.)
Secondly, the individual dog biscuit is described as being "one of which decomposed apathetically in the saucer of milk all afternoon" (29). This description implies Nick's discontent for being with Tom and Myrtle all afternoon. He wasn't interested in being there at all, and in fact wanted to leave. The dog wasn't even interested in being there. It was a sorry situation in Nick's eyes. A forced situation, since someone like Tom (who feels he can call the shots). Almost as though Nick wasn't important enough, in Tom's eyes, to turn down his insistence that he accompany him for the afternoon/evening, although Nick clearly didn't want to, and certainly felt uncomfortable. Yet Tom didn't care.