Hiring a Functional JS developer is beneficial due to their ability to write clean, efficient, and reliable code. They utilize functional programming principles which promote code that is more predictable, easier to test, and debug. These developers are adept at handling complex applications, avoiding side effects, and maintaining codebase which can help in improving the software quality. Their expertise can lead to better data handling, improved application performance, and enhanced user experience. They can also contribute to team learning, promoting practices that can lead to more maintainable and less error-prone code.