This feature is quite similar to Close Friends, one that lets you add posts and stories that only your Close Friends list can see on Instagram. But with Flipside you get to create a different profile altogether. It's quite confusing though for Instagram to introduce this feature alongside Close Friends.
Accounts with a large following usually tend to keep a different private account for their personal posts. But with Flipside and Close Friends, it might lead to more of a confusion, and too many things to handle on one single platform. There's the option to create different audience lists as well so Instagram already gives you enough ways to share posts only with a select group of people.
For now, this feature is only a test with no word on whether it will release for everyone.