A JSON-RPC developer uses the JSON-RPC protocol for designing and implementing software that communicates over a network. This protocol allows for remote procedure calls (RPC) encoded in JSON. Their job involves creating server-side and client-side applications that use JSON-RPC for exchanging data, ensuring the applications adhere to the protocol's standards, testing the applications for performance and reliability, debugging issues, and optimizing the code. They also document their code and monitor the system to ensure smooth operation. They need a deep understanding of JSON, RPC protocols, and the programming languages used in the development process.