An Amplify.js developer utilizes the Amplify.js library to simplify the process of developing JavaScript applications. They use its features for managing AJAX calls, handling client-side browser storage, and implementing Publish/Subscribe messaging for applications. They deal with HTTP requests, manipulate responses, and manage application cache. The developer also uses Amplify.js to create a unified API for various data sources in the application, enabling better scalability. They ensure optimal performance of applications and solve any issues that arise. Their role also involves staying updated about the latest updates or changes in the Amplify.js library to optimize its use.