It stops iOS from saving screenshots of the app when it closes. When an app launches on iOS, it initially draws the UI from a saved screenshot while the app is loading. This creates a privacy risk since that image may contain sensitive info.

Oh, wow, I thought it was some sort of lock screen for the app. There should be a description.

TextSecure has the option to lock the app after a timeout (or on command), and not unlock it until a passcode is given. I suspect Signal has something similar.