it's use case is companies that want to use SOAP but don't want to say they use SOAP
SOAP is in my experience really hard to use and get right, compared to protobufs that bring well understandable set of primitives and intuitive support in many languages. gRPC is a solid carrier for protobufs. Yes, gRPC has many cons (e.g. with undefined/nil values, etc.), but overall it has worked great for our usecases.