Two main situations I think. The first is just interactive use in any shell to encode ad-hoc JSON. If you have a next-gen shell which can handle structured data directly, then you probably don't need it.

Second is situations where you'd rather not add an additional dependency, but bash is pretty much a given. For example, CI environments, scripts in dev environments, container entrypoints. Or things that area already written in bash.

I don't advocate writing massive programs in bash, for sure it's better to turn to a proper language before things get hairy. But bash is just really ubiquitous, and most people who do any UNIX work will be able to deal with a bit of shell script.