Hiring an ImmutableJS developer can be beneficial for your project as they can help in managing your application state in a more predictable manner. ImmutableJS provides advanced JavaScript data structures that promote efficiency and simplicity. They can ensure a more robust codebase by eliminating common bugs caused by mutable data. Additionally, they can optimize performance by reducing the need for data copying and enabling complex data manipulation. Furthermore, their expertise can be invaluable in React or Redux projects, where immutability is a core concept.