Until now, each tool (Glue, EMR, Lambda) does "one thing." But real data pipelines aren't single tasks — they're "receive data from S3 → transform via Glue → pass validation Lambda → on failure send alert → on success load to Redshift," multiple stages linked by order, conditions, and retries. Defining and coordinating this flow is orchestration.
Without orchestration, you trigger jobs manually or via cron and manually dig through logs on failure before rerunning. An orchestrator defines work flow as code, automatically managing dependencies, retries, branching, parallelism, and alerts. AWS offers three main choices, and exams persistently ask "which tool for this situation."