A Flowtype developer primarily works on enhancing JavaScript static typing, thus improving code reliability and maintainability. They utilize Flow, a static type checker for JavaScript, to identify certain types of errors during coding rather than during execution. They annotate JavaScript code with types, which Flow uses to detect errors, ensuring that different parts of the code interact correctly. They also help in code documentation, making it easier to understand the structures and expected inputs and outputs. Additionally, they collaborate with other developers to ensure the software's seamless operation and efficient debugging.