One Piece's Egghead Arc started by immediately separating Luffy, Chopper, and Jimbei from the rest of the crew. For many readers, this feels like a return to the "Straw Hats Getting Separated" trope that's plagued the crew for much of their time in the New World. However, this particular instance of separation provides a golden opportunity for one crew member in, particular-- Jimbei.
Jimbei hasn't got many opportunities as the Straw Hats' newest recruit to demonstrate how he fits into the group. The late Wano Arc shown his fighting prowess but not much more. Isolating him from the rest of the group during the Egghead Arc may be the best method to give him more depth.
Why is Luffy's saviour Jinbei and not Sanji?
The separation happened in Chapter 1061, "Future Island Egghead." On the way to the next island, Luffy and Chopper had fallen overboard and Jimbei needed to jump into the water to save them; he also saved Jewelry Bonney, who happened to be in the general area for unrelated reasons. Unfortunately, a combination of a giant mechanical shark and strong currents prevented the Knight of the Sea from returning to the Thousand Sunny. Thus, he was forced to take the rescued Devil Fruit eaters to the island ahead of everyone else.
This separation from most of the Straw Hats gives Jimbei a chance for some one-on-one time with the main character. He probably would have blended into the crowd if he were with the rest of the crew. With this setup, however, he'll have more of the audience's attention and his actions will feel more meaningful. The same can be said for Bonney.
Sticking close to Luffy for this early part of the arc should also help him to better understand how the crew operates. He'll learn more about the Straw Hats' code of conduct, including their favor of fun adventures over standard piracy and fighting in self-defense. It should also help him realize how much the crew has to reign in their carefree and impulsive captain. Jimbei may have plenty of experience as a pirate, but the Straw Hats are far from the average pirate crew.
As of Chapter 1062, "Adventure in the land of science," Jimbei's role is to help balance out some of Luffy's silliness. Right now, it's mainly Luffy and Chopper doing what they want on this new island. Bonney could keep things on track, but she's also getting caught up in the marvel of the island 500 years in the future. Jimbei helps by providing general information, asking important questions to locals, and keeping the plot and conversations moving forward. In these regards, he's filling a role that might have been taken by one of the other more serious Straw Hats like Nami or Nico Robin.
No longer the Jinbe of the past, He has truly become a part of the Straw Hats ?
It's important to keep in mind the key difference between Jimbei's connections with the Straw Hats before and after joining them. A lot about the Knight of the Sea has indeed been established through previous encounters, including his bond with Luffy. However, his actions before Wano and, arguably, Whole Cake Island were those of an ally. Becoming a full-time contributing member of the main cast should change a lot about how he interacts with the crew and even how he acts in general. In due time, his dutiful and formal attitude could transition into something more friendly and even goofy.
That said, he'll still have to keep his new responsibilities in mind. Before, he was willing to give his life to save Luffy because of his connection to Ace. Now, he has to find it in him to live for Luffy as well as the rest of the Straw Hats. That's what it means to be a helmsman aboard the Sunny.
The Egghead Arc will be the first full arc of Jimbei aiding Luffy not just as another trusted ally but also as a loyal member of his crew. Audiences can look forward to seeing plenty of the former Warlord's integration into the Straw Hat way of life soon.
