I felt like they were putting the viewers in the shoes of Rae's character, by experiencing the same feelings she does with all the humiliating situations she found herself in. It's obvious that she's treated differently because of her size and mental condition, for example her "best friend" at times I didn't like her character because I felt she came across as a bit fake? Like she invites Rae to hang around with her and the gang but the next day no one calls Rae and she goes to the pub to find her best friend who didn't call her and didn't expect her to be there. Rae also had to ask if she was invited to the pool party. They're trying to make an audience see what it feels like for her to not only be overweight, but mentally ill compared to the rest of her friends, and how much harder she finds it to cope with bad things that happen to her like being laughed at and called names over her weight, falling out of the window and getting stuck on the slide in the pool.