Preemption lets a higher-priority device take over as primary, even when the current primary is functioning.
Scenario without preemption: Router A (priority ) fails. Router B (priority ) becomes active. Router A recovers. Router B stays active because it was never "failed."
With preemption: Router A recovers and immediately reclaims the active role because it has higher priority.
Preemption adds a failover event during recovery. Some designs disable preemption to minimize disruptions, preferring to stay on the backup until manual intervention.