You can join on arbitrary json fields, with indices. It is an extremely useful swiss army knife.

I've used jsonb for performing ETL in-database from REST APIs.

I know Concourse from 6.0 uses it for storing resource versions. They had an inputs selection algorithm that became stupidly faster because they can perform joins over JSON objects provided by 3rd-party extensions. Previously it was a nested loop in memory.

https://github.com/concourse/concourse/releases/tag/v6.0.0#3...