The purpose of loading screens is to entertain the user (i.e. don’t let them think the app is hung up) while you’re loading in sounds, artwork, etc. In games like Clash of Clans you get a loading screen while it’s contacting the server and downloading the game state.
Apple encourages the launcher screen to mimic what the user will see when the app does load, so they are discouraging any branding and logo / splash type screens.
If you find your app is being slow to load and your users might think it’s stuck, you probably should consider a loading screen.
Rob