Armchair is called the one in the hallway.
Anyway both of them seem useless for the guide.
Not really.
The armchair (let's call it that from now on since it's gonna get annoying going back and forth everytime) helps you advance time for small increments. Each time of day has 2 "halves". The morning has the 1st half, that one being after getting up, 2nd half of the morning is when Lucile is in the garden and Vera is with the horse. Same goes for Afternoon and Evening, the only exception being Midnight. The armchair helps you advance the time by 1 half each time you sit on it and "Wait", that way you can visit or try to do some stuff only available in specific times.
The chair in your room (as I mentioned, a circle either in or next to the chair) serves the purpose to toggle Beasty on or off, change the MC's name and re-visit some of the scenes in the game, with the exception of the ones that can be repeated ad nauseum (Waking up Lucile with sex, Cosplay, Morning Yoga, etc). This also helps you check in what State each woman is if you don't check the diary.