Binding, over time, makes the breasts appear sagging and empty, quite awful. It gets to the point where the obvious next step is a double mastectomy.
That is the entire reason these binders are pushed -- if you look at any US conference about how to do thing like this for trans kids, they're sponsored by plastic surgery centers and pharma companies.
Many parents would be dead horrified by a child's request to have healthy, functional body parts removed. But destroy their functionality and appearance first via binders, so the parents don't see the damage until it's too late, and they'll say "oh no, I can see how those breasts make you feel awful, of course we'll help you get rid of them." Once they're damaged to the point swimwear and bras don't fit, what's the alternative?
Go have a look-see at photos of breasts that have been bound "safely" long term. You'll see exactly how binders are the thin end of a wedge designed to get parents to buy cosmetic surgeries they would otherwise object to.
I'd be terribly worried if I had a daughter who did this that she was experiencing bullying or even sexual abuse. In many studies, over 1/3 of girls expressing a desire to transition genders had experienced sexual abuse.