A Bower.js developer manages front-end packages and dependencies for web applications. They utilize Bower.js, a package management tool, to handle the installation, updating, configuration, and uninstallation of different libraries, frameworks, and utilities used in a project. These may include jQuery, AngularJS, and Bootstrap, among others. The developer ensures the correct versions of packages are used and resolves any conflicts between dependencies. They also maintain the bower.json file, which tracks the packages needed for an application. Their role is crucial in streamlining the development process, improving efficiency, and ensuring the application runs smoothly.