Stateful apps require a stable hostname rather than a pod name with a random string.

Using an index, your pod name will have a persistent identification. For example, podname-0, podname-1, and podname-2 (and when the pod is rescheduled, it retains that identification).

The volumes associated with stateful sets will not be deleted if a statefulset is deleted or scaled down (preserving data)

A statefulset enables your stateful app to locate other peers by using DNS.

Cassandra and Elasticsearch clusters utilize DNS to discover other cluster members.

Using statefulsets, you can operate three Cassandra nodes on Kubernetes, numbered cassandra-0 through cassandra-2.

If you did not use statefulset, you would receive a dynamic hostname, which cannot be used in a configuration file since the name can change at any time.

statefulset also allows your stateful app to order the startup and tear down

When scaling up, it goes from 0 to n-1 (n=replication factor) When scaling down, it starts with the highest number (n-1) to 0