Poké Ball Plus works like a Joy-Con™ controller with the Pokémon: Let’s Go, Pikachu! and Pokémon: Let’s Go, Eevee! games for Nintendo Switch™. Move, battle, navigate menus—you can do it all with your Poké Ball Plus.
Just like with Pokémon GO Plus, you can use your Poké Ball Plus when playing Pokémon GO on a compatible smartphone to help you search for Pokémon.
Source: The Publisher Website.