The Loading Screen is a pre-game screen where summoners wait for everyone's game to be ready to start.
The summoner can also see its ping. So while the loading screen is active, he/she can solve problems before the game starts.
He/she can use its computer even if the loading screen is in full screen, the user can use
Alt + Tab on Windows to move to other windows. If there's not any other window, he/she can use
Windows + D to get to the desktop.