Controversial Zoro purpose in the story is to make Luffy looking good.

[Toonpiece]

In the very earlier chapter of one piece. Luffy said it will bad look if future pirate king don't have greatest swordsman under him. It will make himself bad. Zoro purpose on the story is to make Luffy the future pirate king good. That is Zoro promise to Luffy but he broke that promise against mihaw and he broke it many times. His captain promise and kuina promise is essential for Zoro. That's the reason why oda purposely make rocks,Roger,

mihawk,shanks,Oden, Rayleigh a swordman. Cause one day Zoro will surpassed all those
guys and be under Luffy. It make Luffy good. Its like buggy and mihawk situation. Mihawk greatest swordman make buggy look good. Same can be said to Zoro and Luffy except Luffy is not fraud like buggy. Luffy fans and Zoro fans. We need to have peace with each other with top 1 and 2 eos agenda.

 

[HA001]

Lik would die before he made peace with luffy. He still has king > Luffy

 

[RoronoaZorro]

Then after those first few chapters until a thousand chapters later, Sanji is drawn and given poorer treatment all the time, just to make Zoro looks better in comparison.