Answer: Through pointing out the differences between the classes of society and how Myrtle is desperate to be a part of it.
Explanation: Myrtle is desperate to get as far away from her depressing life with Wilson at the gas station as she can, surrounding herself with the material trappings that Tom can provide: an apartment, clothes, and an accessory dog.
In a novel that is all about the American drive to get ahead, Myrtle is one of the strivers, willing to put up with terrible treatment in exchange for a chance to climb higher. So are the people hanging on her coattails, like the McKees and Catherine. Seeing her with this shows us just how striated (separated into layers) society is, as Myrtle grabs every tiny opportunity to demonstrate her slightly higher status to her entourage.
