Why does Fitzgerald add the detail about the dog-biscuit to the description of the party at Myrtle's apartment in The Great Gatsby?

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?