A load balancer is a piece of software that routes the web traffic from clients to different backend servers. It dispatch the network traffic to many backends (generally) so each backend has to handle only a fraction of the global traffic. The load balancer also detects when a backend service is unhealthy and ensures that all requests are dispatched to healthy services.
Using SquareScale, a load-balancer is necessary to give quality access to your application, scheduled anywhere on your high availability cluster. It also helps reduce downtime, should a host or a service instance fail. During deployments, it helps keep the service up for all your customers.