A json-simple developer works with the json-simple library in Java for encoding or decoding JSON text. They use it to parse JSON text into Java objects, or 'serialize' Java objects into JSON text. They are tasked with managing and manipulating data, often from APIs, in the JSON format. This includes creating, reading, updating, and deleting data. The json-simple library provides methods to convert JSON strings to Java objects and vice versa, making it simpler to handle JSON data. Therefore, a json-simple developer's job involves ensuring smooth data interchange between systems using this lightweight and efficient tool.