As you can see in the above sequence, posted on Twitter (and shared by Matt Navarra), the new option is activated when you go to exit out of the Stories composer mid-process. When you choose to save a Stories draft, a new alert will notify you that Stories drafts get deleted a week after saving them. So they won't be there waiting for you forever, but the option does provide you with additional scope for putting together more comprehensive, crafted compositions, which you can then share at the best time for your audience.
The feature could be particularly handy for social media managers who are looking to post at optimal times - though it is also worth noting that you have kind of been able to save and share stories, in different ways, for some time. Up till now, all users have been able to save their Stories by either downloading them to their device or via some third-party apps. But those tools are not native to IG and are generally not as convenient within your creation flow. By having your drafts within the app, you'll be able to view them as they'll appear to users, while you'll also be able to utilize Instagram's full feature set in your draft creation process and post on the go via the app. Social Media Today asked Instagram for more info on the Stories drafts roll out, and it provided the following statement:
|"Instagram Stories drafts are now available for everyone, globally. Story drafts will save for seven days before disappearing."|