The function of a load balancer in web traffic management is to distribute incoming network traffic across multiple servers to ensure optimal performance and reliability. By preventing any single server from becoming overwhelmed, the load balancer enhances responsiveness and minimizes downtime. It acts as an intermediary, intelligently directing user requests based on various factors, such as server health, current load, or user geographic location. This not only improves application availability but also enables seamless scaling by adding or removing servers as necessary, thus accommodating fluctuations in traffic.