What an interesting question! Let's examine the detail a bit more closely before we begin the discussion:
Mrs. Wilson was first concerned with the dog. A reluctant elevator-boy went for a box full of straw and some milk, to which he added on his own initiative a tin of large, hard dog-biscuits--one of which decomposed apathetically in the saucer of milk all afternoon. (Fitzgerald 29)
Now, although this is inherently an opinion question, I believe that this part about the dog-biscuit is relevant because it reiterates once again the despair, apathy, immorality, darkness, poverty, and sadness that revolves around the entire Valley of Ashes, . . . all of which the eyes of T. J. Eckleburg look over with brooding, vacant eyes. It is in direct contrast with the glittering munchies served in East Egg in Chapter 1 as the "old rich" snack in their own type of apathy. The decomposing dog-biscuit simply adds to the squalor and disgust in the entire scene with people getting drunk amid clouds of smoke. The whole scene leads to this telling description where the dog is involved yet again:
The little dog was sitting on the table looking with blind eyes through the smoke, and from time to time groaning faintly. People disappeared, reappeared, made plans to go somewhere, and then lost each other, searched for each other, found each other a few feet away. (37)
The sad thing is that, by the end of the novel, the reader is left wondering this: Who is a more pathetic example of humanity? The inhabitants of the Valley of Ashes, . . . or of the rich in East Egg or West Egg?
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.