The primary purpose of a load balancer in web applications is to distribute incoming traffic across multiple servers to ensure reliability and optimal performance. By managing the workload, load balancers can prevent any single server from being overwhelmed, enhancing response times and uptime. They also provide redundancy, improving fault tolerance in case of server failure. Load balancers can be hardware-based or software-based and are essential for scaling applications to accommodate fluctuating user demands and maintaining a seamless user experience.