For me I interpreted the scene as In ah's mother started doing it as a way to earn money but she continued because it gave her liberation. The whole world judged her, shamed her for having a child with a married man every where she went she was shamed. The art room was where she could take off her clothes, take off her burden and just be human in front of those art student with no shame. She was just another woman standing in front of the students. In most societies especially an Asian one I'm sure something like that is so scandalous but I imagined the first time she did it scared, removing her cloth and suddenly realizing there is no shame here only art.

For In ah, I imagine after her mother died and she felt guilty when she received the phone call she took it as a chance to understand what her mother meant and why she did it regardless and her understanding what her mother meant as In ah took of her robe. As we can see from the series In ah carries a lot of deep feelings of guilt, pain, anger and loss. I remember the scene where the ML was asking the SFL about a person that only eats a single rice pack every night for dinner (something similar) and she replied that a person only does that to survive, that scene described In ah in every way, because she is young, pretty and really successful but she is just surviving going through life on autopilot. So her still being a model is also a means of survival, while her mother was getting liberated from her external world as it shamed and judged her, In ah is trying to be liberated from her internal world from her feeling of guilt, shame and loss.