Data dumps and exports between systems. REST APIs, databases, that sort of thing. CSV is the most common utf8/ascii option.

'pletnes provided some use case classes. JSON is an ubiquitous data interchange format, so document sizes vary with data. I'll chime in with a real example I look at daily: the compilation database of a C++ project I work on, i.e. the compile_commands.json file. This file is fed to a language server (clangd) so it can correctly process the source code, and provide IDE features to text editors. The JSON file itself, as created by CMake, is some 2+ megabytes in size. Once run through `compdb` utility, which adds missing entries for header files, it grows to 25 megabytes.

I'm converting proprietary AutoCAD DWG files to some JSON format (libredwg) I invented. 600K bytes is a very small file, with just a couple of objects.

Many other CAD formats also switched to JSON.

My Spotify playlists export is a 7.5MB (minified) JSON and I don't even have many of them. Cleaned up of redundant entries and formatted it comes out at over 128K lines.

JSON logs would be obvious example here

machine learning datasets are often in JSON and can be arbitrarily huge