Its what makes the meta game interesting. Imagine if all Manectric had was electric moves, you'd be able to easily predict its moves and send in an appropriate counter such as Landorus or Ferrothorn. But with more type coverage for weaknesses, Manectric could run Overheat and HP Ice to force the opponent to put more thought into their turns. Also, it prevents the game from being mundane and using the same moveset. Pretty sure I was the first person (or one of the first) to start the Aerial Ace Gliscor here on Reborn