Full screen mode allows an application window to use the entire screen. In this mode, the GNOME 3 top bar and any application bars are only displayed in response to user action. This allows whatever content is being displayed in the window to be given the maximum amount of attention.
Full screen mode can be provided whenever activities are immerse or require a high level of attention (watching videos or document writing are good examples). In cases where someone is likely to want full screen mode, it is appropriate to automatically enable it. Video players can and should switch to full screen mode when someone selects a video for playback, for example. In cases where full screen might be useful, but is not the primary way in which content is viewed, full screen can be provided as an option (this is appropriate for document editing).
Closing a content item should disengage full screen mode.