The function of a load balancer in web architectures is to distribute network or application traffic across multiple servers to ensure no single server becomes overwhelmed. This enhances the performance and reliability of applications, improving user experiences by minimizing delays and preventing outages. Load balancers can manage incoming requests, direct them to the least busy servers, and perform health checks to remove unhealthy servers from the rotation. Overall, they play a critical role in optimizing resource utilization and ensuring high availability for web applications.