The idea of transience informs nearly every aspect of Breakfast at Tiffany's. It's the desire to remain unfettered and unchained that propels Holly and that drives much of the action of the story, and there's a definite sense of impermanence throughout the narrative. We get the feeling early on that the relationships and connections that develop won't be long-lasting, that they are matters of convenience more than anything else. And this take on relationships creates a general feeling of anxiety since we're always waiting for the other shoe to drop. In a world marked by transience, it's hard to know what to hold onto (both for the characters and for us as readers), and this impacts how we read the novel since it puts us on edge.
The idea of transience applies just to the physical settings in the novel.
Transience operates on a symbolic level. It's indicative of a larger, metaphorical theme in the novel.