The Poke Ball Plus is a device that players can purchase that can only be used in connection with this game. Unfortunately, it cannot be used as a controller as it could with Pokemon Let's Go Pikachu & Let's Go Eevee. However, you still can receive the Mythical Pokemon known as Mew via the Mystery Gift feature with the help of the Poke Ball Plus. On top of this, players can send the Pokemon they catch within the game to the Poke Ball Plus in order to take them on a stroll.
As you walk with one of your Pokemon within the Poke Ball Plus, you will earn items by doing a variety of things and gaining points. Your Pokemon within the Poke Ball Plus will randomly call out to your trainer through vibration of the device, and if interacted with in time, you will earn points. You can even play with your Pokemon in the Poke Ball Plus by holding down the control stick for two seconds in order to initiate Play Mode, earning points by gently shaking or rotating the device after doing so.
Additionally, the Poke Ball Plus can connect to Pokemon GO, and you can earn even more points by doing stuff with it within said game. The more points you accrue with the Poke Ball Plus, the more Item Rewards you will receive in Pokemon Sword & Shield when the Pokemon is returned. In total, there are five different Reward Items you can receive: Poke Ball, Great Ball, Ultra Ball, Star Piece, and Rare Candy.
We have questions and answers related to this topic which may also help you: