The significance of this compilation may be a bit subtle, even to Pokémon fans. But Pokemon Diamond and Pearl, which launched in 2006 on the Nintendo DS, was the last edition of the game to have encounter animations for the Pokémon bestiary. Later editions of the game did not use these encounter animations.
So this compilation by Master of Hyrule, is an archival resource, of all battle encounter animations for the 494 Pokémon extant as of Diamond and Pearl (with their battle cries). We've featured MoH's work before — such as this collection of all the faeries in The Legend of Zelda.
Master of Hyrule said this took "tons of hours. A few days to do." The least you can do is watch for a few minutes, right?