You can resize a cluster by increasing or reducing the number of nodes. In most cases, you want to expand a cluster, adding nodes so the cluster capacity matches the growth of your data. However, you can also choose to reduce the number of nodes in a cluster. Rebalancing then redistributes shards evenly across all eligible (non-excluded) nodes.
You do not have to rebalance the import or API nodes after resizing the cluster. You just have to restart the tier after resizing so all the nodes in the tier are used correctly.