As well as saving your chosen filter (and edits) the draft will also save your caption so far so it’s perfect for pre-writing posts for later.
Activated on most accounts (update your app if it’s not appearing) the new feature works by hitting cancel like you normally would, to avoid posting a new pic. When you hit the final screen a slide-up menu will appear giving you a new option, however to save your work for later.
From there you’ll notice a ‘Drafts’ section above your latest camera roll snaps next time you go to post something brilliant. Pre-edited and captioned from before. Life saver no?