Hiring a Marionette.js developer can significantly enhance your web application's functionality and user experience. They possess expertise in this robust framework that simplifies complex User Interface issues, making your web application more efficient and interactive. With their knowledge in backbone.js, they can create scalable and maintainable applications. They can also provide solutions for memory leaks and zombie views, ensuring smooth and seamless performance. Additionally, their skills in event-driven architecture can help in creating versatile and modular applications. Overall, a Marionette.js developer can bring a lot of value to your project.