clustering provide hierarchical topology support in routing protocol instead of flat topology. In cluster based routing protocol the cluster head coordinates all the routing activity. So cluster node have to maintained only topology information instead of all the node in flat topology . so in this way clustering reduce the control overhead for route formation and maintenance.
The role of cluster head should be regularly rotated among the nodes in a cluster (say, based on the residual energy); this way, cluster-based routing can be both energy-efficient and scalable. With node mobility, it is better to prefer nodes with higher residual energy and lower mobility as clusterheads.