We need to support over 10M files in each folder. JSON wouldn't fare well as the lack of indices makes random access problematic. Composing a JSON file with many objects is, at least with the current JSON implementation, not feasible.

CDB is only a transport medium. The data originates in PostgreSQL and upon request, stored in CDB and transferred. Writing/freezing to CDB is faster than encoding JSON.

CDB also makes it possible to access it directly, with ranged HTTP requests. It isn't something I've implemented, but having the option to do so is nice.