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>More challenging than casual play
>Less monotonous than previous Battle facilities
>Exp gain = best post-game training
>NO use of any items (besides already held items)
>Multiple areas which progressively get tougher
>Interesting boss
>Shiny Pokemon as gift
It was just such a great way to train post game and challenging enough to actually be fun.
Even if you don't intend to train, you still do so while having another goal in mind