What purpose does the Dark Cave serve in Pokemon HeartGold/Soulsilver?

Question

There is nothing "Out of the ordinary" in the cave except for Dunsparce.

Answer 1

Inside Dark Cave you can find a unique item, the Blackglasses, which raises Dark-Type attack power. Also you can find some ordinary items in the cave. More importantly, you can find rare Pokemon like Wobbuffet and Dunsparce in the cave.

Answer 2

It may hold some rare stuff.