It's to explain to Readers why Luffy said that he cannot accept less than becoming World's Greatest Swordsman from Zoro
Because he needs a Bodyguard stronger than him
PS: Whenever Zoro fights alongside Luffy, he takes the Spotlight in most times:
1. They both fought Morgan, Oda chose to let Zoro be the one who Finish him off
2. They both fought PH Dragon, Oda chose to let Zoro the one who kills it
3. Zoro & Luffy got attacked by Senor Pink, Dellinger & Machvice, Oda chose Zoro to be the one who handles them quickly
4. They both met Fujitora, Oda chose Zoro to be the one who stops him before Pica appeared
5. Both faced Hawkins, Oda chose Zoro to be the one who handles him
6. Both faced Apoo, Oda chose that Zoro carries & protects Luffy
7. Both are facing 2 Yonko now, well guess who is protecting the other one multiple times...
Not to mention that when a Major Foe appears who isn't Main Arc Villain but Equal or Greater Threat, Zoro is the one who handles him such as Mihawk in Baratie Arc, Kuma in Thriller Bark, Fujitora in Dressrosa & Stampede Movie ... etc
Because he needs a Bodyguard stronger than him
PS: Whenever Zoro fights alongside Luffy, he takes the Spotlight in most times:
1. They both fought Morgan, Oda chose to let Zoro be the one who Finish him off
2. They both fought PH Dragon, Oda chose to let Zoro the one who kills it
3. Zoro & Luffy got attacked by Senor Pink, Dellinger & Machvice, Oda chose Zoro to be the one who handles them quickly
4. They both met Fujitora, Oda chose Zoro to be the one who stops him before Pica appeared
5. Both faced Hawkins, Oda chose Zoro to be the one who handles him
6. Both faced Apoo, Oda chose that Zoro carries & protects Luffy
7. Both are facing 2 Yonko now, well guess who is protecting the other one multiple times...
Not to mention that when a Major Foe appears who isn't Main Arc Villain but Equal or Greater Threat, Zoro is the one who handles him such as Mihawk in Baratie Arc, Kuma in Thriller Bark, Fujitora in Dressrosa & Stampede Movie ... etc