The point is to deploy immutable images that were clean rebuilt from scratch and tested prior to deployment, rather than upgrading an environment which becomes risky after some time. Another added value is automatic actions based on container watchers, traefik for example will self-configure on the fly when you just spawn a container with the hostname in a label. If deployment becomes that easy then why not leverage gitlab dynamic environment feature and deploy on $branchname.ci.example.com so that you can have say a product owner to review the development prior to merging to master (which would deploy to staging if you have an aggressive CD strategy - which I do)