XML wraps data in opening and closing tags, with attributes and nested children. It powers RSS feeds, Atom, SVG, sitemaps, Office documents (`.docx` is a zip of XML), and most enterprise SOAP APIs.
It's much more verbose than JSON, but it has schemas (XSD), namespaces, and built-in transformation (XSLT), features JSON took years to grow.
For new web APIs, prefer JSON. For interop with existing systems, you'll still meet XML.