If the websites which are to be hosted have a heavy traffic due to which it is not possible to host the websites on one single server, Clustered servers are required.
Cluster basically involves a number of servers (minimum 2 ;)) which can vary as per the requirements of the websites. The load of the websites is then distributed on the servers so that no server within the cluster has excessive load.
The other benefits of Cluster involves high availability which means that even if one of the servers in the Cluster fails, the other servers will manage the load of the websites
