Being a Parisianne is making most of one’s circumstances, which, here, are Paris: high-density urban living, among benevolent snobs and a handful of friends; small apartments; and a subtle equilibrium in "what we do with what is done to us” (Sartre's definition of freedom).
Sometimes, in order to be X, one must not be born X. It is only by consciously deciding to be X that one can confidently inhabit X.
Why would one want to be a Parisian wherever one is? One may naturally be a Parisian to begin with, and it is reassuring to know one is not alone in one's constructed world. One may also wish, if not to be someone else for a while, then at least to be aware of alternative ways of being.