Does Immerse Gaming Software work with FFXIV. If yes, why do I need to buy a separate license for Immerse Gamepack FFXIV i.e. what are the additional benefits?

No, Immerse Gaming software will not work for Immerse Gamepack app. As Immerse Gaming software relies on a surround sound output from your game, whereas Immerse Gamepack app renders full 360 personalized spatial audio of objects directly inside the game, which gives you the additional benefit of exact positions of objects in the game.