I googled the two word together but could not find anything. I have never used it. Is there any necessity for it although we have main.storyboard.
The launchscreen.storyboard (basically a loading screen) is what comes up when the user loads the application. How long it appears to the user depends on how long it takes for the user to load your application. You can change the duration of the launchscreen.storyboard but it is not recommended, generally users want the app to load as fast as possible
The main.storyboard is appears onto the screen AFTER the launchscreen.storyboard