Munander Singh explains why you might want to use gRPC for cross-process communications:
When the client invokes the service, the client-side gRPC library uses the protocol buffer and marshals the remote procedure call, which is then sent over HTTP2. Server un-marshal the request and executes the respective procedure invocation using protocol buffers. The response follows a similar execution flow from the server to the client.
The main advantage of developing services and clients with gRPC is that your service code or client side code doesn’t need to worry about parsing JSON or similar text-based message formats (within the code or implicitly inside the underlying libraries such as Jackson, which is hidden from service code). What comes in the wire is a binary format, which is unmarshalled into an object. Also, having first-class support for defining a service interface via an IDL is a powerful feature when we have to deal with multiple microservices and ensure and maintain interoperability.
The “yes, but” here is that based on your language of choice, gRPC can be a bit tricky to get going.