If you ordered the Guilty Gear 20th Anniversary Pack from Europe or Japan, you would see this happen when inserting the game card:
Instead of having to pick between the two games in-game, they let you choose from the Switch menu.
What benefits does this give?
It allows you to make collections of games that have already released on the eShop and put it into a single game card like the Psikyo Collection which is now up to its third volume:
With regards to the Guilty Gear Anniversary Pack, since Guilty Gear and Guilty Gear XX Accent Core + R are listed separately on the eShop. You can choose to buy only one game instead of spending more to own both if they were released as a pack on the eShop.
Instead of having to pick between the two games in-game, they let you choose from the Switch menu.
What benefits does this give?
It allows you to make collections of games that have already released on the eShop and put it into a single game card like the Psikyo Collection which is now up to its third volume:
With regards to the Guilty Gear Anniversary Pack, since Guilty Gear and Guilty Gear XX Accent Core + R are listed separately on the eShop. You can choose to buy only one game instead of spending more to own both if they were released as a pack on the eShop.