A Mocha Test Suite developer writes and maintains tests for JavaScript applications using Mocha, Sinon, and Chai. Mocha is a JavaScript testing framework providing structure for writing tests, while Sinon allows for test doubles like spies, stubs, and mocks, enabling behavior verification. Chai is an assertion library used to check results. The developer uses these tools to ensure code is functioning as expected, prevent regressions (bugs that reappear after being fixed), and facilitate debugging. They work closely with other developers to understand the functionalities to be tested, and they update tests as codebase evolves.