why is the physical setting of the play described in such careful detail

Williams’s production notes and stage directions emphasize his innovative theatrical vision. He felt that realism, which aimed to present life as it was without idealizing it, had outlived its usefulness. It offered, as Tom puts it, “illusion that has the appearance of truth.” Williams sought the opposite in The Glass Menagerie: truth disguised as illusion. To accomplish this reversal of realism, the play employs elaborate visual and audio effects and expressionistic sets that emphasize symbolic meaning at the expense of realism. To underscore the illusions of the play, Tom makes a point of acknowledging these devices during his monologues as narrator.

The Wingfield apartment in St. Louis.

Tennessee Williams makes a big deal out of telling us all about the apartment. He wants us to know how the buildings are all stacked up like a beehive, so we get the sense of dehumanization and confinement to working roles. Because the action ONLY takes place at the apartment, we can sense Tom’s feelings of being trapped, the fact that he is contained in only one location along with his family. The fire escape, of course, is crucial, being a means of escape and all. It kind of hangs out there like a constant foreshadowing of Tom’s eventual escape. It’s also, fittingly, the place where narrator Tom does a good deal of his narrating. This makes sense – narrator Tom has already escaped, so he speaks to us from outside the apartment.