https://www.scientificamerican.com/article/the-mindset-of-eating-disorders/

Looking in from the outside, it can be difficult to understand an eating disorder. Why would anyone want to throw up, starve themselves, binge until they hurt, or feel tortured by food?

But eating disorders serve a purpose for those who suffer from them. After all, Psychology 101 teaches us that behavior exists because it gets reinforced. Therefore, once we understand what individuals derive from their eating disorders—how bingeing, purging, or restricting meets a need—it makes way more sense.