Android 4.4 (API 19) introduced a UI feature called Immersive Mode. Unlike the previous full screen flag, your app receives all the touch events while in Immersive Mode. This mode is ideal for certain activities, such as reading books and news, full-screen drawing, gaming, or watching a video. There are several different approaches to full screen, and each have a best use case:
Reading books/articles, and so on: Immersive Mode with easy access to the system UI
Game/Drawing app: Immersive Mode for full screen use but minimal system UI
Watching video: Full screen and normal system UI
The key difference between the modes is how the System UI responds. In the first two scenarios, your app is expecting user interaction, so the System UI is hidden to make it easier for your user (such as not hitting the back button while playing a game). While using full screen with a normal system UI, such as watching a video, you wouldn't expect your user to use the screen at all, so...