Because the backup instance has no impact on live users, it is best used for any changes or updates to the production instance that should be tested. Changes that are successfully tested on the backup can then be repeated on the production instance in order for them to actually reflect onto the live system.
You can be logged into the backup and live instances simultaneously, however, it is recommended to dedicate one browser to live, and one to backup. That way there is less chance of making an unintended change in the live instance by selecting the wrong tab, and you can test your changes in multiple browsers.
The backup is also a great resource to allow new internal users who are unfamiliar with the SmartSimple system to learn how to navigate and learn.
The backup instance does not send any emails triggered by workflows, but you can review the output results in the In Queue tab of the Message Queue, so it is an excellent environment to safely test workflows.