Somebody added protobuf (because it was “faster”). The only thing faster is RTT because of of smaller payload, and that can be fixed with gzipped JSON.
The part that is distasteful to me is the relative work to maintain protobuf (compared to JSON). I use java so we use Jackson object mapping for JSON. Adding to a response means adding a field to a POJO.
In contrast, for protobuf I have to modify the .proto file, then update the serialization and deserialization logic which moves the data in/out of the protocol buffers and into POJOs. It has happened more than once that a release has happened where someone added a field and forgot to put in either the serialization or deserialization code for the protocol buffers.