It makes depression more relatable to someone who's never had it. Instead of depression being the character's character, it's described as something she just can't shake -- like a disease, as it is.
All the people I've ever known who were suicidal felt the same way as was described in her suicide letter; as if it was just their time to go. They felt no desire to go on because they were kept down by their disorder.
When Fiona's depression is separated from her in her note, it created more empathy for her. She's not just weepy because she feels like it and that's the way she is; she's weepy because she's utterly hopeless and overcome by a disorder mostly beyond her control. She's not a bad mother just because that's how she wants to be, it's because she can't get herself up to be the best she can -- her true self.
Did Fiona's letter make you have more sympathy for her, too?