I have a project which can work on 3D objects imported from STL files. My file format has more metadata and much more detailed information w.r.t. a standard STL file, and all the data and the metadata can be written in a way which is both readable and modifiable by a human if needed be.

Having the same tags many times means the file can be nicely compressed, it's being XML means it can be verified independently with a schema (and the schema can be defined as a remote location over HTTP if needed be), too.

You can always store data more efficiently with binary formats, but XML DOM parsers allows to access arbitrary parts of the tree instantly, so working with it is both easy and fast at the same time.