While Pokemon Legends: Arceus makes many substantial changes to the traditional Pokemon formula, both in exploration and combat, there are still lots of mechanics and features that remain exactly the same. Like in previous Pokemon games, players can check their Pokemon's friendship levels in Pokemon Legends: Arceus. As to why a player would need to check if a Pokemon is happy or not, it's because there are specific Pokemon that can only evolve by increasing its happiness levels to a certain amount. As for the Pokemon that evolve through high friendship/happiness levels, some examples are Riolu and Chansey, which can evolve into Lucario and Blissey, respectively.