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


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.




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.

