The biggest plus of using integrated registry is that you have integrated authentication and authorization of GitLab that follow your groups and members assigned to your GitLab projects, making it really easy to have private container repositories stored on registry.

Second the built-in registry is really easy to configure and maintain. You have to specify the address and provide a certificate to start using it. You also use the same backups mechanism as you use for your GitLab installation.

In the future we may make it possible to use external storage for container images, like S3. This is something that is already supported by docker/distribution.