Because it's best for health and well-being. Check out this video featuring Dr. Jennifer Guadiani, MD a physician who specializes in treating those with eating disorders https://youtu.be/E2isludJPnM. She explains why this model is best for all - not just those with eating disorders.
Health at Every Size has many facets, but in a nutshell your health is complex and isn't based on how you look or what you weigh. Here's a few places to start learning about HAES®: