I don’t think it’s to fit in with the guys I just don’t think they had proper boob support for the job she’s doing. Her boobs would be all floppy and irritating while fight and running with out proper support and I doubt they got proper heavy duty sports bras she needs I can’t even find a right one. Binding secures them and keeps them out of the way which is most convenient and I doubt they know about the health risk in an era where they still wear corsets.
I didn't mean fit in with the guys in at "she wants to be a guy" since. I mean that every story I've read with a female warrior that doesn't present herself in a traditionally feminine way is depicted binding their chest.
And they knew about some of the heath risks, simply because its hard to wear a real corset comfortably. Like it restricts movement and your ability to breathe. Women passed out while wearing them. So on a surface level they knew somethings. They just didn't care.
There are other undergarments for women back then other than corsets. They were simple garments, made out of linen, so not as "sexy" and easy to romanticise, but they existed.

Yeah I know ...real life and fantasy don't mix. But she should stop. Not because she needs to embrace her womanly figure, but because binding in that way (with just bandages) can put pressure on her ribs and blood vessels. It can also cause breathing problems. I mean, once again, I know this is set in ye olden times so they don't have proper chest binder and compression shirts that are available to people today. And I know that every fantasy female warrior trying to fit in with the guys binds her chest that way. But since I know it can be detrimental to peoples health so it bothers me lol.