What purpose does the Varsouviana Polka serve?

The Varsouviana polka is used by Williams to highlight themes of death. This music is diegetic only for Blanche – when we hear the polka, we are hearing what is inside her mind. This memory worms its way to the forefront of her consciousness when she is recalling her husband and when she feels emotionally threatened, and serves to highlight her disintegrating sanity. Like the "click" that Brick awaits while drinking in Cat On A Hot Tin Roof, Blanche must wait for the polka to play out till the gunshot that ended her husband's life before she can shake off the auditory hallucination.