Some people oppose the premise for ZKK because they think it collides with what luffy does as main character of this series. However, ZKK actually benefits luffy.
The story in one piece is getting wrapped up, and we are about to see the long awaited reunion between luffy and shanks. This is what luffy said back at the first chapter.
Pay attention to how luffy stated the promise is for his crews to be stronger than shank's. Therefore, the strawhat crew needs to do something beyond what red haired pirates can do. And this is where ZKK plays its role.
Zoro killing the world strongest creature is the indisputable moment where the strawhat crew is shown to be better than even the pirate emperor crews, red haired pirate. Ergo, ZKK is an important milestone for luffy too.