Why?
As someone who has found themself needing to manually write and programatically work with data serialization formats, I wanted a better way. I tried many formats: JSON, YAML, TOML, CSV, XML, KDL, Lua tables, Java properties, and others. You name it, I tried it. Many of them had enough nagging issues to cause motivation in me to find a better format, which never arose.
"But JSON works fine"
Have you ever been in the position of writing JSON, rather than just having a library parse it? If you haven't; then yes that's a logical conclusion. Personally, I find myself in a position where I need to write data manually, and many of the popular formats make that experience have more friction than it should. For those that may need a data serialization format, but never (or rarely) have to deal directly with the data in its storage format, it may seems like nit-picking; however it becomes different when you find yourself manually writing in these formats.