(Part of the) setting for a new minigame/sidequest/subquest/whatever you want to call it. You'll have to solve a murder mystery in the mansion, the murderer, murder weapon and motive are randomly assigned each time you start the game with all the dialogue changing accordingly, so there should be a fair bit of replay value. If I may say so, I think this minigame will be longer and offer more gameplay than many complete games published on here (it's things like that that make this game take forever to finish).
The mansion has a second floor, basement, park and four little annexes (including the classic green salon where you can gather all the suspects to accuse one of them). The gates will lock once the murder mystery starts, you won't be able to leave until you've solved the murder case.