XML is in a totally different category than JSON: it's a tool to craft formal languages. A data format or a config is also a language of sorts, so it can do tha

XML is in a totally different category than JSON: it's a tool to craft formal languages. A data format or a config is also a language of sorts, so it can do that as well, but it's real purpose are languages in general. XML languages are what is left of a language if you omit parsing and start with AST. Configs and data formats normally don't require all the benefits of a language (e.g. composability), but other things do, e.g. UI systems.