People implementing public-facing APIs might not be comfortable forcing all the consumers of that API to use Protobuf, but may want to use Protobuf internally w

People implementing public-facing APIs might not be comfortable forcing all the consumers of that API to use Protobuf, but may want to use Protobuf internally within their service. So, Protobuf lets you convert to/from JSON, so that your service can expose a JSON API while maintaining Protobuf schemas and never dealing with a JSON parser directly. The implementation is based on Protobuf's existing reflection features, so if you are already compiling with that enabled, the JSON implementation does not add much code bloat on top of that.