What is the main purpose of Portgas D. Ace's death?

Question

I want to know what turn of events did it take by the death of Portgas D. Ace in One Piece. I don't seem to understand.

Answer 1

Out of universe reason: the capture and death of Ace seems to setup the War of the Best and ensure that Luffy unequivocally lost it. This moment of loss is like "The Ordeal" in "The Hero's Journey". Think the Empire Strikes Back for Luke. By having the main character lose and recover, it deepens the character and makes what comes next more meaningful.

This arc is analogous to the Loguetown arc. Both arcs are the last ones in their particular sea before the short arc to enter the new one, reintroduce characters from earlier in that sea, and introduces some characters who will be important in the future. More importantly, they show some characters who are so overpowered compared to Luffy it is hard to see how he will beat them and has Luffy lose that fight. This shows Luffy needs to get stronger in the next sea quickly or he will die.

In Loguetown, Luffy is the one on the exection platform. He has to be saved by his daddy, loses a fight to an enemy he can't lay a finger on, and runs away. He even needed help to run away. This teaches the audience that the Grand Line will be far harder than the East Blue.

In the War of the Best, it is worse. Luffy plays the role of the lightning bolt to release Ace from the Execution platform but is proven to be no where near strong enough to protect Ace. Even to get that far Luffy needed the help of strangers. This clearly shows the audience what the new World will be like. It also motivates Luffy to get stronger...once he recovers his composure.

Answer 2

So far, as I remember, Ace death are impact to some event on One Piece time line.

-From this, It's revealed about Luffy's backstory about how he met and began his brotherhood with Ace is shown.
-Luffy and Ace's foster mother is revealed to be Curly Dadan.
-In the flashback, his second brother Sabo who is presumed deceased is shown.
-In the flashback, Sabo's parents adopt another boy in the case Sabo would fail named Stelly who, as revealed in the Totto Land Arc is now the King of the Goa Kingdom.
-The S crossed out on Ace's arm is not an error but a homage to Sabo.
-During this flashback, Bartholomew Kuma is seen with Dragon and Ivankov as a revolutionary.
-Sabo's talk with Dragon would foreshadow his return in the Dressrosa Arc and his involvement with the Revolutionary Army.

And also from this,

-Ace is killed by Admiral Akainu, the third admiral to be introduced, which causes Luffy to fall into a coma until he recovers in Post-War Arc. Before his death, Ace mentioned Sabo, who is introduced in the flashbacks Luffy has after he awakes, and later plays a major role in the story.
-As revealed in the Dressrosa Arc, Ace's death would have an effect on Sabo who unknown to both Ace and Luffy at the time was still alive. As a result this helps Sabo recover his memories after being stricken with amnesia for ten years. It is also the reason why he is so determined to protect Luffy so he would not have to lose another "sworn brother".
-After Ace dies, Whitebeard predicts that someone will "come to inherit his will". This prediction later comes true as Sabo eats Ace's Devil Fruit after he dies and specifically voices his desire to inherit the late pirate's will, mirroring Whitebeard's prediction exactly.
-Ace's death would cause his Devil Fruit, the Mera Mera no Mi, to resurface later and be in the possession of Donquixote Doflamingo in the Dressrosa Arc as a means to lure Luffy to him.
-Because of Blackbeard murdering Whitebeard, the remnants of the Whitebeard Pirates would later fight the Blackbeard Pirates one year after the war at Marineford. The Whitebeard Pirates suffered a complete defeat and have not been seen since.