The answer is different when it comes to Zoro because Luffy DOES fight, because his dream (despite people trying to overcomplicate it) INDEED INCLUDES becoming the strongest (Roger was the strongest as Whitebeard was the only one known to rival him, Chinjao stated surpassing Roger would mean surpassing the admirals and the emperors and becoming the king among the kings, etc.). Luffy doesn't cook, doesn't build, doesn't heal..., but he fights, and he has been getting the strongest enemy consistently for more than two decades while Oda didn't take the trouble to create a second Crocodile, a second Lucci, a second Doflamingo... for Zoro to defeat someone on his level.
Luffy is the main character and virtual chosen one in a shonen manga who bases his development on fighting and defeating main villains. If you think he isn't the strongest within his crew then you are clearly overcomplicating this story aimed for teens and lying to yourself.
Zoro is the combatant because he doesn't know how to do anything else and anybody should see that. And that's why, when it comes to role, he's the only one inferior to his captain.
ALL the Crew fight & ALL of them need & want to get Stronger so this is nothing new, as i said it's just Journey Requirement to get stronger, it's not their Goal. Zoro's Desire to fight Kaido shows that he doesn't only aim for Swordsmen, he aims for anyone who considers himself Strongest.
Since all the Crew Fight & need to get Stronger, then Oda could have not made the "Fighter" Role & made Zoro the Shipwright or Historian or something. But nope, he only gave him "Fighter" Role & "Strongest" Dream so pls stop switching definitions to match your wishful thinking
No one considered Roger Strongest man, stop changing Facts:
WB was called Strongest Man & Danger wise, he was more Powerful than Roger due to having a DF capable of tilting the whole Oceans.
Shiki told Roger to become his Right Hand Man. Mont d'Or said that even Roger can't escape BM Pirates Army waiting near the Mirror (Which was an Overestimation ofc) but they also said that No One can stop Big Mom (Which Oda later confirmed in SBS that whole Totto Land must flee when BM goes Crazy because no one can stop her) so BMP never believed Roger is more Dangerous than their Mother.
Kaido was also called Strongest Creature & in 1 v 1, always bet on him.
So Roger was never considered & mentioned by anyone to be Stronger than WB, Shiki, BM or Kaido.
He is simply the Man who can Rival them all, survive entire Grand Line & find One Piece & that is what Pirate King means.
And i gave you a list why Luffy fights the Main Villain dude, it's because he is Main Character.
Let me ask you, imagine you created a Main Character & gave him a Stronger First Mate & you want to draw the Main Character crushing the Dreams of Villains, so how do you make sure the First Mate doesn't Fight instead of Main Character?
You obviously have 4 Options:
1. Make Villain a Bad Match Up for Zoro & can only be Fought by Luffy
2. Make Zoro Weakened & Unable to Fight so Luffy has to step in
3. Make Zoro not meet Villain at all & is busy doing something else
4. Make Zoro fight someone Stronger than Main Villain who is a Secondary Character in the Arc
Which is EXACTLY what Oda done in every single Arc. Oda doesn't need to create a Second Crocodile or Second Lucci or Enel ... etc because this isn't about just Action, it's about Story & Oda wants to Portray Luffy as the one who Dreams Bigger & have the most fearsome Ability in the World which is Uniting People & making them Ally with him & Help him. (That's why he is the Next Joy Boy, not the Next Ryuma who was both Strongest Swordsman & Man)
Whole Crew are aimed for the Audience man.
Readers can relate to one of the SHs when they read One Piece, this is what Oda created different than other Shonen, instead of making One Inspiring Character, he made Ten (So far) & each teaches a different lesson
Luffy inspire in Leadership, Friendship & Will to never Give Up & always Smile ... etc
Sanji inspires in Heroism, Chivalry, Kindness, Treasure Food ... etc
Robin inspires in being Mature, Reserved, Treasure History, let Others help ... etc
So what do you think Zoro is made to inspire Readers to?
It's all about Fighting, Getting Stronger, Training, Hard Work, Enduring Damage ... etc
You say Zoro is the Combatant because he doesn't know how to do anything else, so basically he is USELESS, right?
Because all other Straw Hats Fight too, so it's like saying Zoro is Useless
When someone who never read One Piece asks you about Crew Roles, you will tell him:
Sanji cooks & Nami navigates & Chopper heals & Robin studies ... etc & Zoro just fight?
They will obviously ask you, so other Crew Members don't Fight? And you will say No, they all Fight & get Stronger
Then that means Zoro has No Role according to your Logic & everyone who never read One Piece will be confused by your Answer
Zoro's Role means he is Strongest Fighter in the Crew & that is what makes him stand out from the rest just like they all stand out in something.
And plz go read Chapter 01, Luffy himself stated his Goal, it's about Leadership, not being known as Strongest unlike Zoro.