A Flask application developer designs and builds web applications using the Flask framework, a Python micro web framework. They create server-side logic, implement front-end elements into the application, and connect the application with other web services. They ensure the responsiveness and efficiency of the applications, maintain data storage solutions, and integrate user-facing elements developed by front-end developers. They also troubleshoot applications and code for various platforms to ensure optimal functionality. Furthermore, they collaborate with other team members to define and design new features, while also maintaining code integrity and organization.