It depends. If you're a big manga fan and love having a collection, they can be totally worth it. You get multiple volumes in one go and can enjoy the series without having to hunt for individual issues.
In most cases, 'The Girl in a Box' is not a true story. It might be a fictional tale designed to explore various concepts or to provide an interesting and captivating narrative for the audience.
Since there are likely different 'girl in a box' real stories, without more context, it's difficult to say who the girl was. It could be a victim of a local crime that received media attention in a particular area, or it could be part of a more widespread and international incident. Maybe she was a young girl who was kidnapped by a group and held in a box - like a wooden crate - as part of their criminal operations.
I'm not sure specifically which 'girl in the box' true story you are referring to as there could be multiple cases. But generally, it was a girl who became a victim of some sort of cruel situation that led to her being in a box.
It's possible for manga box sets to be fake. Sometimes, poor-quality reproductions are made and sold as the real thing. This can be difficult to spot for the average buyer.