Hot on the heels of One Piece's "Impel Down Arc" was the "Marineford Arc," a climactic battle in which the famed Yonko Edward "Whitebeard" Newgate lost his life in battle. However, for the sake of One Piece's story, this was an essential development in many ways.
As one of the four Yonko and the captain of the Whitebeard Pirates, the pirate played a major part in the world's balance of power, and unsurprisingly, his death tipped the scales at once. With him gone (as well as his subordinate Portgas D. Ace), the Marines claimed a major victory, as did the many pirates who dreamed of plundering Whitebeard's many territories. Blackbeard also scored a major victory in taking out his rival and even (by unknown means) stealing Whitebeard's Devil Fruit power, becoming a terrifying powerhouse (and the newest Yonko) in the process. But Whitebeard didn't die with regret, despite all this.
Whitebeard was a pirate, not a king or emperor, and he knew full well that the world of pirates is based on a predatory "might makes right" system, no matter who you are. He was the strongest player, but he had many enemies who wanted him gone, and no boundaries or balances of power last long in a world like One Piece's. That's what makes it so fun for many pirates, who can easily grab any power they can through violence. And in all fairness, anyone else could do the same to them, as per the unspoken rules. This isn't a world about entitlement or divine right to rule; the current top dog is just the most recent winner of these pirate feuds, and no one stays at the top for too long.
Not even Gol D. Roger could stay on top forever, and like Whitebeard, he died with a smile on his face, not tears in his eyes. He and Whitebeard both knew that everyone loses this game eventually, and so they bowed out with grace. So, to further develop Whitebeard's character and embody the spirit of piracy, One Piece creator Eiichiro Oda needed to off this powerful character and prove that no pirate is truly invincible.
But Whitebeard's death did more than reinforce the story's themes of dog-eat-dog piracy: it also threw open the doors to a whole new story arc. It's a matter of opinion whether the story was starting to get tedious or stale by that point, but Oda kept things fresh with Whitebeard's demise and the ensuing effects on the world. Many new players were now free to step onto the stage and make their mark, whether by conquering major islands, competing for the position of Yonko, or hunting for the One Piece. Naturally, the Straw Hats pirates were game too, and while Luffy dearly misses his foster brother Ace, he made the most of both Ace's and Whitebeard's sacrifice in that battle.
This also served as a time for the major players to reassess their own positions and power, and Gecko Moria, arguably the weakest of the Warlords, was stripped of his title. It was believed that in the face of the coming New World, his power was inadequate, and that made room for someone new to take his place. That also ties into the theme of dog-eat-dog, where positions of power are earned and have to be defended tirelessly. Now, Luffy and his crew are facing new enemies while old ones are left behind, and they are going everywhere from Fish-Man Island to the Japanese-inspired Wano to face greater foes than ever before.
And without Whitebeard's symbolic and world-shaking sacrifice, that might have never happened at all.
