Hey,
SSH setup is required to do different operations on a cluster such as starting, stopping, distributed daemon shell operations.
Hadoop core requires shell i.e, (SSH) to communicate with slave nodes and to create the process on to the slave nodes. The communication will be frequent when the cluster is live and working in a fully distributed environment.
For our single-node setup of Hadoop, we, therefore need to configure SSH access to localhost for the Hadoop user.