Within Pokemon Legends: Arceus, there are a multitude of sidequests players can take. NPCs with a black/white Poke Ball icon over their head indicate they have a Request for trainers. As the main Missions/story of Pokemon Legends: Arceus progresses, more Requests become available. Each Request has different rewards. While they're not necessary to complete the game, completing them can be beneficial as most of the rewards can be used when catching Pokemon.