STONITH (Shoot The Other Node In The Head aka. fencing) protects your data from being corrupted by rogue nodes or unintended concurrent access.
Just because a node is unresponsive doesn’t mean it has stopped accessing your data. The only way to be 100% sure that your data is safe, is to use STONITH to ensure that the node is truly offline before allowing the data to be accessed from another node.
STONITH also has a role to play in the event that a clustered service cannot be stopped. In this case, the cluster uses STONITH to force the whole node offline, thereby making it safe to start the service elsewhere.
