Pokemon Magikarp card market price has dropped to 1/4

I heard it was trading for over a hundred thousand won just a few months ago
But it looks like it's trading for under 40,000 won now
I happened to get one and sold it for 60,000 won on Karrot Market,
which made me realize this kind of market exists,
but I'm curious about what they are used for and why they were so expensive in the first place