A load balancer is essential in web server architecture as it evenly distributes incoming client requests across multiple servers. This helps ensure that no single server becomes overwhelmed, which can lead to performance degradation or server failure. By balancing the load, it enhances the availability, reliability, and scalability of applications. Additionally, load balancers can perform health checks on servers to ensure only healthy services receive traffic, and they can facilitate SSL termination, further optimizing performance. Overall, their function is crucial for maintaining a seamless user experience.