A cluster is a network of queue managers that are logically associated in some way. Clustering is available to queue managers on all WebSphere MQ V5.3 platforms.
In a WebSphere MQ network using distributed queuing without clustering, every queue manager is independent. If one queue manager needs to send messages to another it must have defined a transmission queue and a channel to the remote queue manager.
If you group queue managers in a cluster, the queue managers can make the queues that they host available to every other queue manager in the cluster. Then, assuming you have the necessary network infrastructure in place, any queue manager can send a message to any other queue manager in the same cluster without the need for explicit channel definitions, remote queue definitions, or transmission queues.
There are two quite different reasons for using clusters: to reduce system administration and to improve availability and workload balancing.
As soon as you establish even the smallest cluster you will benefit from simplified system administration. Queue managers that are part of a cluster need fewer definitions and so the risk of making an error in your definitions is reduced.
For details of all aspects of clustering, see WebSphere MQ Queue Manager Clusters.