I can understand why other productions used a different set for Great Comet, but there's something so perfect about Pierre being in the pitt with the director. He watches as life goes on through a literal barrier between him and the rest of characters, down bellow them. He sees Natasha from afar, listening to her story as it happens but as an spectator, never near her. He only accompanies them from a distance, through his piano, but not with his presence.
When he enters the scene though, everyone notices it. All eyes are on Pierre as he sings and even direct the other characters sometimes still from the pitt. His first attempt to get out, The Duel, ends up with him realizing that he needs that company, to get out, but is not quite ready to do so. In The Abduction, he takes some time to join the others and their celebration, but he ends up fitting and dancing with his fellow man. But in Call to Pierre, he's finally ready to join the action and become a full character, away from the piano and the pitt. I think it's a really beautiful way to show his journey.