Answer
During a sync, ArgoCD orders resources by waves and runs hooks (PreSync/Sync/PostSync) at the right phases, waiting for each wave's resources to become Healthy before proceeding. So health assessment gates progression: if a resource in an earlier wave isn't Healthy, later waves and PostSync hooks won't run.
Real-world example A database migration PreSync hook must complete Healthy before the app's Deployment in the next wave is applied.