It matters to build more lore around the King vs Zoro duel.
Have King be the one to kill Ushimaru before he can even make it to Kaido, and then have King get direct control over the region of Ringo and put it's people through all the hardship they've suffered up to this point

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Ti's what it is.
It was clear from the get go that King and Queen are Zoro's and Sanji's opponents, and Oda denied the haters the chance to downplay Sanji because "he is getting help"