Megiddo is significant because this is where the last great battle before the Millennium takes place. Only this time, even enough overwhelmingly outnumbered, Jerusalem prevails. The WTS believes Armageddon will be a worldwide conflagration of good v. evil, in which Jehovah comes to the Earth and judges the people and nations. All the righteous will be resurrected and live in the paradisical glory enjoyed by Adam and Eve in the Garden of Eden, whilst all the wicked will be destroyed or resurrected and then destroyed.
In that day shall the Lord defend the inhabitants of Jerusalem; and he that is feeble among them at that day shall be as David; and the house of David shall be as God, as the angel of the Lord before them. And it shall come to pass in that day, that I will seek to destroy all the nations that come against Jerusalem. I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplications: and they shall look upon me whom they have pierced, and they shall mourn for him, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his firstborn. In that day shall there be a great mourning in Jerusalem, as the mourning of Hadadrimmon in the valley of Megiddo.
So why is that? Why is there a great mourning in Jerusalem?
The bad guys have just killed your best two assets, you're defenseless. You have no more weapons to unleash and then you suffer a major earthquake! Then you look heavenward and see a man descending out of the sky. It's the promised Messiah (or Jehovah, your call)...finally! What's there to be mournful about? Shouldn't this delivery from one's enemies be a celebratory event? But then there's that part about looking upon "me" whom they had "pierced."
What would happen if the Jews ran into the valley to escape their enemies, and when they looked upon their Messiah, much to their astonishment, found themselves gazing on Jesus of Nazareth, the one whom their fathers slew by the hands of the Romans? The scripture mentions it would be like mourning for one's "only son" with a "bitterness" akin to someone losing "his firstborn."
Yet the WTS misses the point entirely. As they explain it:
But how do they know that the place where the warring parties gather is not literal? Isn't prophecy literal by definition? If a prophet says something will come to pass at a certain place and time, what would it prove if the prophet's followers came back later and said, "Well of course it didn't happen that way! You took me literally! It actually did happen in 1912 in Pittsburgh, the spiritual Jerusalem!" Well, it would make the prophecy a laughing stock.
Simply stated, the war of Har–Magedon is the future earth-wide battle in which Jehovah’s appointed King, Jesus Christ, will destroy God’s enemies. The Bible explains that these enemies, “the kings of the entire inhabited earth,” are mobilized by “expressions inspired by demons” and are gathered together “to the war of the great day of God the Almighty...to the place that is called in Hebrew Har–Magedon.”—Revelation 16:14, 16.
The place where the warring parties gather is not literal. The name Har–Magedon, rendered “Armageddon” in some Bible translations, means “Mountain of Megiddo.” (Revelation 16:16, footnote) There has never been a mountain that bears that name. Furthermore, “the kings of the earth and their armies” could not literally gather in one place. (Revelation 19:19) Rather, “the place” refers to the condition or situation into which earth’s political rulers and their supporters are led—one of opposition to Jehovah and to “the armies that were in heaven” under the military command of the “King of kings and Lord of lords,” Jesus Christ.—Revelation 19:14, 16.