WARNING: The following contains spoilers for the One Piece manga, available in English from Viz Media.
With the Big Mom and Kaido fight in full swing, One Piece's current Wano arc is the midst of a cataclysmic event after reaching its 1,000th chapter milestone. Given that Luffy has plenty of strong fights over the course of his pirate life, though, one might wonder what makes the fight against these two Yonko any different? Well, we could potentially see sweeping changes to this world should Luffy and his allies come out as the victors.
The obvious difference between Kaido, Big Mom and Luffy's previous adversaries are that the two larger then life pirates are Yonko. As Emperors of the Sea, these two have ruled over parts of the New World for decades. A Yonko's influence and power are not to be underestimated. The Navy was only prepared to fight Whitebeard and his crew when they executed Ace. What ended the war was the arrival of a second Yonko, the Red-Haired Shanks. Continuing the fight would only end in massive casualties on both sides, a risk Fleet Admiral Sengoku saw was not worth taking.
Furthermore, Kaido and Big Mom have decided to become ally pirates. The last time these two worked together was 38 years ago in the Rocks Pirate Crew. For them to suddenly work together again could spell untold devastation for the world of One Piece.
Like the best One Piece villains, their alliance has been foreshadowed many years in advance, foreshadowing that tells the audience that they've made marks on this world that can not easily be erased. For instance, although we meet Shanks at the start of One Piece, we don't truly learn of the Yonko status he holds until the end of Water 7 when Koby and Garp inform the Straw Hat Crew of what's awaiting them in the New World. After that, we see them mentioned off-handedly here and there throughout various One Piece arcs.
Another example is Gecko Moria, Captain of the Thriller Bark Pirates. His original crew was completely wiped out by Kaido when he first fought the Emperor of the Sea in the New World. This is what drives him to create an immortal pirate crew of zombies that are easily defeated, but it's also why he puts more stock in having his crew work for him rather than being a selfless captain for them. Kaido has had lasting effects on various individuals and locations across One Piece -- he's not some two-bit clown who's taken over a single port village.
As for the actual fight taking place in Onigashima, it's been made explicit every fight and every role is essential to defeating Kaido and Big Mom in the long run -- not just Luffy's. Kaido has over 30,000 pirates in total. Some of them, like the Tobiroppo and All-Stars, rival Charlotte Katakuri (Big Mom) in strength. It's vital that characters like Robin, Brook, Franky, Sanji and Jimbei are able to defeat their respective foes at the moment. Anyone that could give more support to Kaido and Big Mom while Luffy and Co. are fighting them is a risk that cannot be allowed to continue. This is why Chopper curing the Ice Oni Virus and Tama using her special Dango to befriend the Gifters, though seemingly small, is, in fact, critical, since the Raid team is still vastly outnumbered by the Beast Pirate forces.
But as this raid is playing out, a small force has bunkered down in Onigashima to watch the chips fall where they may, which is another clue as to this event's wider significance. The elite intelligence-gathering shadow organization, CP0, is hiding within the walls of Onigashima, watching the war play out and keeping score of who's winning this fight. If Kaido and Big Mom are to be defeated, that'll mean the three supernova captains fighting them -- Luffy, Law and Kidd -- could become Yonko themselves. Furthermore, for three rookie pirates to take down two of the oldest names in the Golden Age of Piracy could have disastrous effects on the world as a whole, possibly inciting a new boom of pirates to invade the Grand Line.
It's clear from all of this that the battle at Onigashima is the most seismic event in One Piece since the Paramount War. No matter who ends up as the victor, news of this battle will still send a shockwave throughout One Piece's world as a whole. It's scary to imagine what will be waiting for the Straw Hat Pirates will all this is over.