For gRPC, the protobuf is the API specification, a service definition with endpoints, what requests to those endpoints should look like and what responses look like. Of course, there are better and worse implementations, e.g. a well commented proto definition explaining what various args do, etc.
In gRPC the definition is a requirement to use, so at the bare minimum you have the typed structure of requests and responses. There is no such requirement for REST
In gRPC the definition is a requirement to use, so at the bare minimum you have the typed structure of requests and responses. There is no such requirement for REST