The key advantage of using a content delivery network (CDN) for web services is improved performance and reliability in content delivery. CDNs cache content in multiple geographically distributed locations, or nodes, which allows users to access data from the nearest server. This reduces latency, minimizes load times, and alleviates bandwidth strain on the origin server. Furthermore, CDNs enhance security features like DDOS protection and provide redundancy, ensuring that content remains accessible even during high traffic or server failures.