Question

What benefits do we get by changing Arceus with plates?

Answer 1

Arceus doesn't evolve, it changes type. His stats don't change, his nature doesn't change, nothing changes except his type. This is beneficial largely to his synergetic role on a team. If you're weak to Water it will be useful to have Arceus-Water (Splash Plate) or Arceus-Grass (Meadow Plate) on your team.

Thanks to his large movepool it also allows him to fit on a variety of team archetypes such as stall or offense.

Comments

Ok, MK you pretty much have the answer, but you forgot one thing
When Arceus changes plates, the move Judgement also changes type to the corresponding plate(Like if Arceus holds the Flame Plate Judgement will become Fire type) That's it I guess.

Hope I helped!

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For small details comment please this alone wasn't enough for a second answer.