How powerful is XML

It's no different from using a .wz or a .package

In fact, most of those now days are just containers to bind and compress multiple XML files or files or various formats.

There honestly is no real advantage to it other than it's an actual documented format with libraries to read and write to it and standards.
It's no more or less effective from writing out everything to flatfile in a proprietary format or directly serializing it to disk.

It's just a means of saving it dynamically outside of the executable.

I'm not familiar with libraries, flatfiles and direct serialization. What are those, and how do they factor into things?

I'm also not sure what a proprietary format is.

EDIT: You can write programs with excel and a calculator? How? If it's a graphing calculator I think I might have an inkling, but a normal calculator? And excel?