All 3 acts are set in the Birling's living room
(1) Euphemises their self absorbtion how they think of nothing outside of their own lives.
(2) Symbolises their disconnectedness from the wider world.
(3) Helps give a claustrophobic mood, as when the inspector arrives it mirrors the idea of an interrogation room.
(4) Priestley allows us to focus on the main point of the play, the moral lesson.