When you have a site or service on a single node and the traffic requires you to scale horizontally. So then you have two or more nodes for the same service. You put them behind a load balancer which then sends each request, say, to one or the other, in a round robin or some other fashion.
A secondary advantage of this is that if one of the nodes goes down for some reason, the load balancer can automatically take it out of service with minimal impact to users.
