That, and having the format definition file support comments means that you have a convenient all-in-one place where, at lest in the more straightforward cases, you can handily describe both the message format and the service's behavior in a single file that's easy to share among all your teams.

Precisely.

The easiest path with JSON is to do none of this, and so the majority of teams (particularly inexperienced ones) do none of it. With protos, someone must at least sit down and authoritatively outline the structure of any data being passed around, so at a minimum you’ll have that.

But even just forcing developers to do this generally means they start thinking about the API, and have a much higher chance of documenting the semantics and cleaning up parts that might otherwise have been unclear, confusing, or overly complex.