And the access controls of DNS services are often (but not always) not fine-grained enough to actually prevent someone from ignoring the procedure and changing every single subdomain at once.
It does help. For example, at my company we have two public endpoints:
company-staging.com company.com
We roll out changes to company-staging.com first and have smoke tests which hit that endpoint. If the smoketests fail we stop the rollout to company.com.
Users hit company.com
Then you can update the DNS configuration for company-staging.com, and if that doesn't break there's very little scope for the update to company.com to go differently.
The scope for it to go wrong is the differences in real-world and simulation.
It's a good thing to have, but not a replacement for the concept of staged rollout.