This is basically because it stores PUBG mobile files on the device in the form of pictures and sounds data. This happens in every application/game. In PUBG these files include the splash screen, the profile icon, the avatar, the skin for clothes, guns, gears and so on. The game also downloads and stores files for every map. The benefit from this is that you will not have to download the entire 2GB game data and the 500MB data of each map every time you open the app. In addition, this allows for in-game download of screen pictures when you get a title or a reward.