In the case of the former, its because the best choices vary not only by color identity, but often by specific deck building limitations: Seismic Assault and many other red effects want you to play an extra-high land count, while a black deck with reanimation and Tortured Existence obviously cares about discarding creatures
You can also double up on Gifts Ungiven with Azula, and grab a ton of pieces for the Breach combo while putting others in the graveyard
Base sets are often smaller, in the range of 60 to 80 cards where an English set might run past 100, and the content that eventually becomes a single English set is sometimes split across a couple of smaller Japanese releases
Break Open will trigger those effects
Because of the incredible illustrations, they are wanted by many collectors across the globe
Dryad Arbor Among the most unique lands in the game, Dryad Arbor is notable for being the games only true land creature hybrid