A Balsamiq Mockups developer designs and creates user interfaces for software or websites. They use Balsamiq, a rapid wireframing tool, to create mockups or prototypes that simulate the final product. These mockups help developers and clients visualize the layout, design, and functionality of the software or website before actual development begins. They can also iterate and refine these mockups based on feedback to ensure the final product meets the user's needs and expectations. Their role involves close collaboration with other developers, designers, project managers, and clients. They may also conduct user testing to validate the design.