I don't think it's unique to Korean boards, although it may still be more popular there.
I remember my first two keyboards (early 1990s) had the large Enter and small Backspace. I have two keyboards now that still have it from the same era (KPT-102 and Chicony KB-5181).
It does take a moment to adjust but isn't really a big problem. I, too, don't really see any kind of advantage of this setup though.