The email job has been failing silently for three months. 50,000 emails not sent. Or the background sync has been silently skipping records. Or the backup has been succeeding at creation but failing at upload. Silent failures are the most dangerous kind.
You deploy to all instances simultaneously. A bug affects 5% of requests. Before you can react, 100% of users are hitting it. Canary deployments let you catch that bug when it''s hitting 1% of traffic, not 100%.