It is a metaphor for Lily's situation because her situation at the pink house will not “stick” much longer.
It is likely that Tom picks Myrtle because he thinks she will be an easy woman to control. She is not wealthy, powerful, or socially connected, so she doesn't pose much of a threat to him. Myrtle also readily accepts Tom's fabricated story about being unable to divorce Daisy because "she's a Catholic."
