A Moq developer uses the Moq library in .NET applications for unit testing. They create mock objects to simulate the behavior of complex objects and isolate the unit of code they are testing. The developer sets up expectations on mock objects, such as specifying return values or checking if particular methods are called, providing a controlled environment for testing. They ensure that the code functions as expected and debug any issues that arise. The Moq developer's role is crucial in maintaining code quality, reliability, and enhancing the application's performance by identifying and fixing bugs early in the development cycle.