As we already know that Author preview mode is just to check whether styles and authored content is working as expected before publishing.
However we have some funtionalities that requires publish instances such as end user login or securing pages etc.,
If you want to verify those type of specific functionlaities which requires publish environment those can be done using the preview environment.
This is one of the uses of using preview, we can also use this for content validation by the content approvers, design validations by UX team and finalize everything in preview instead of publishing them directly.
Hope this is helpful.