Cluster:
A logical grouping of one or more functionally identical application server processes. A cluster provides ease of deployment, configuration, workload balancing, and fallback redundancy. A cluster is a collection of servers working together as a single system to ensure that mission-critical applications and resources remain available to clients.
Clusters provide scalability. For more information, refer to additional documentation that customer support may provide that describes vertical and horizontal clustering in the WebSphere Application Server distributed environment.
Above is the explanation for WebSphere cluster. In WebSphere world, a cell can have one or many clusters. I want to know in which case one application should be deployed in more than one cluster in WebSphere?
You cannot deploy exactly the same application to more than one cluster, if you need more processing power you just add members to the cluster.
One of the few reasons that comes to my mind to deploy to second cluster, could be to use different application version - check this post for more details and restrictions of deploying multiple versions of the same application.