The Big Mom Pirates, for instance, use marriage to strengthen their already incredible numbers while the Whitebeard Pirates had 43 allied ships with him when he graced the seas around the Navy HQ in Marineford. With each passing arc, the Straw Hat Pirates close in on the Yonko, and just like the rest of them, having a large allied force is necessary if Luffy is to be included among the greatest pirates in the world, which is exactly where the Straw Hat Grand Fleet comes in.
Eiichiro Oda, the author of One Piece, sowed the seeds for the birth of the Straw Hat Grand Fleet immediately after Luffy entered the Corrida Coliseum to win Portgas D. Ace's Mera Mera no Mi in the Dressrosa arc. Being a Coliseum, the place was filled with mighty fighters from all over the world such as the legendary pirate Don Chinjao of the Kano Kingdom's Happo Navy, Bartolomeo the Cannibal from East Blue, Cavendish of the White Horse from the Bourgeois Kingdom, to a name a few. Although they were Luffy's enemy at the Coliseum, every single one of them ended up being drawn to him when the fight against Doflamingo broke out.
Cunning as he was, Doflamingo planned to have every single fighter in the Coliseum turned into toys to add to his underground weapons trade and while most of them did fall prey to his scheme, it was Luffy's pirate crew who eventually saved them, making all of them indebted to him. Soon, the revolt against Doflamingo began and with the help of the Grand Fleet, the Straw Hat Pirates eventually overcame a difficult ordeal by taking down the Donquixote Family once and for all. Immediately after the arc, the Straw Hat Grand Fleet was born as every single pirate captain from the Corrida Coliseum pledged their loyalty to Luffy and exchanged sons' cups with him.
The Straw Hat Grand Fleet consists of 5,640 pirates and is a major force in the One Piece world that makes Luffy one of the "Great Pirates," according to Bartolomeo. So far, the fleet appears to have a total of 7 ships, with Cavendish leading the First Ship, Bartolomeo leading the Second Ship, Don Sai leading the Third Ship, Ideo leading the Fourth Ship, Leo leading the Fifth Ship, Hajrudin leading the Sixth Ship, and finally, Orlumbus leading the Seventh Ship.
Immediately after the formation of the fleet, however, they went their own ways and have, since then, continued to make waves in the pirate world. At the same time, Luffy has traveled to Whole Cake Island and clashed with a Yonko, and then headed to Wano to take down Kaido. Both of these almost feel like the perfect opportunity to make use of the Straw Hat Grand Fleet, after all, what could be bigger than taking down two Yonko? Interestingly, the Straw Hat Grand Fleet hasn't even been mentioned so far, which clearly indicates that they are to serve a much bigger purpose in the story moving forward and it is one that Oda has already hinted at in the Dressrosa arc.
While fighting the Yonko feels like a task that would require the Straw Hat Grand Fleet to step in and provide their assistance to Luffy, Oda clearly isn't heading that way and the signs were clear from the very beginning. In the Whole Cake Island arc, fans saw Luffy join forces with another member of the Worst Generation, Capone 'Gang' Bege and seek help from the New Fishman Pirates to deal with Big Mom's forces. In One Piece's Wano Country, on the other hand, the Ninja-Pirate-Mink-Samurai alliance appears to be proving itself enough to help take down Kaido and Big Mom. Clearly, the implication is that the Straw Hat Grand Fleet is being saved for something even bigger than the Yonko which, while a huge claim, does make a lot of sense.
Heading back to Chapter 800 of One Piece, titled "Son's Cups," the Straw Hat Grand Fleet pledge their allegiance to Luffy and the Straw Hat Pirates and as they do so, the narrator's box says all one needs to know about their future role. It is made clear that the Straw Hat Grand Fleet is far from being at full power at the point. In fact, the narrator goes out of his way to say that from there onwards, each crew will go on to make a great name for themselves and gain further strength in the pirate world. Once they do so, the Straw Hat Grand Fleet are going to cause a massive incident of "historic proportions." Evidently, the Straw Hat Grand Fleet won't be called into action anytime soon because their role is to assist Luffy in the biggest war to come — the Great War that Whitebeard, Oden, and Roger spoke of.
After the Wano Country arc, Luffy will already have established himself as a Great Pirate in the world, and with 3 out of 4 Road Poneglyphs in his possession, Laugh Tale would almost be within his grasp. With Luffy being close to finding out the true history of the world and, at the same time, his friends like Vivi and Hancock, and his brother, Sabo, being in danger, an all-out war against the World Government and the Navy is inevitable. This is exactly where the Straw Hat Grand Fleet comes in to support Luffy as he can't possibly take on the biggest force in the One Piece world without help.
In the Marineford arc, it was made abundantly clear that the Marines hold the most manpower in the world with around 100,000 men. With just 11 men, taking on such a massive force would be impossible. Luffy will inevitably end up needing the Straw Hat Grand Fleet as well as many other allies of his, such as the Fishmen, the Kuja, warriors from Wano, and maybe even the forces of the Revolutionary Army.
For the Straw Hat Grand Fleet, the goal would be to ensure that Luffy brings about the major change that the world is waiting for. Luffy is the man who will become the next Joy Boy and in doing so, bring a smile to the entire world. With the threat of the SSG, the Gorosei, the Navy Admirals and Vice-Admirals, and the SWORD organization, the help of the Straw Hat Grand Fleet will be pivotal. As always, they will cut down whoever stands in Luffy's way and open up a path for him to topple the corrupt World Government. Together, the fleet and the Straw Hat Pirates will save all those who Luffy holds dear and then unite the world just as Joy Boy is destined to do one day.