The key advantage of functional programming in software design is its emphasis on using pure functions and immutable data, promoting cleaner and more predictable code. This programming paradigm allows developers to focus on the *what* rather than the *how*, leading to fewer side effects and easier debugging. Functional programming also enhances parallel processing capabilities, making it well-suited for applications that require concurrency and scalability. Ultimately, it increases code maintainability and can lead to fewer bugs in complex systems.