iOS documentation states that a launch image is a requirement for App Store submission, however I currently have an app available that does not have one (simply forgot to add it) so it would seem it is something they are not overly strict about enforcing or the documentation (that I read, at least) is incorrect
The image itself is shown while the application is initiated. You can control the duration by making it so that your app launches faster (or slower, though I do not know why you would want to do that).
It is required so that the OS has something to show while the app initializes (since the system cannot screenshot the app and use that as it does when switching between apps)
