The “View Once” feature deletes photos or videos from your chat after they’ve been opened by the recipient. For instance, while sending a photo or video, you can make it view once by tapping the “1” button to the left of the send button.
When the recipient opens it, it’ll be deleted, as he/she will be able to see that it’s a disappearing photo. This means that it’s the type of thing you’d only want to use for people you trust to not take a screenshot.
In June, Mark Zuckerberg asserted that the feature would be coming, though details about when it would release were scarce.
As an example of how the feature could be used, WhatsApp uses the example of sending a photo of sensitive information, like a Wi-Fi password. Indeed, there are perhaps other less wholesome uses for this kind of feature.
But either way, it’s useful to have an option between the app’s disappearing messages, which currently only go away after a week, and your chat’s permanent record.