The seventh bowl is significant for several reasons. It is the conclusion and final expression of God's wrath. Since the first introduction of the seven angels with the seven plagues, the apostle John sees that they represent the last of God's wrath, and when the last bowl is dispensed, someone very near God says, "it is done."

Then I saw another sign in heaven, great and marvelous, seven angels who had seven plagues, which are the last, because in them the wrath of God is finished. (Rev 15:1)

Then the seventh angel poured out his bowl upon the air, and a loud voice came out of the temple from the throne, saying, "It is done." (Rev 16:17)

When one examines the consequences of the seventh bowl (Rev 16:17-21), there are a number of harmful events that take place before the seventh plague is released at the very end.

Overshadowing the seventh plague of monstrous hailstones is the great earthquake that precedes it. This same earthquake is associated with the sixth seal:

I looked when He broke the sixth seal, and there was a great earthquake; and the sun became black as sackcloth made of hair, and the whole moon became like blood; and the stars of the sky fell to the earth, as a fig tree casts its unripe figs when shaken by a great wind. The sky was split apart like a scroll when it is rolled up, and every mountain and island were moved out of their places. (Rev 6:12-14)

Because the sixth seal stands unique in causing an event on earth without an angel mediating God's wrath, it appears that the apostle John is recording the consequence of breaking the sixth seal, which is comprised of the successive angelic mediated events of the seventh seal, seven trumpets and seven bowls. By culminating in the catastrophic earthquake, the sixth seal appears contemporaneously as the seventh bowl.

With the release of the last plague of huge hailstones, which is not mentioned in the sixth seal, the seventh bowl resumes the narrative sequence of the apostle John's prophetic revelation.

It may be interest to note and compare the reactions of human beings during the sixth seal and seventh bowl.

After the great earthquake:

Then the kings of the earth and the great men and the commanders and the rich and the strong and every slave and free man hid themselves in the caves and among the rocks of the mountains; and they said to the mountains and to the rocks, "Fall on us and hide us from the presence of Him who sits on the throne, and from the wrath of the Lamb; for the great day of their wrath has come, and who is able to stand?" (Rev 6:15-17)

After the huge hailstorm:

And huge hailstones, about one hundred pounds each, came down from heaven upon men; and men blasphemed God because of the plague of the hail, because its plague was extremely severe. (Rev 16:21)

During the course of the dispensation of the seventh bowl, there is a reference to "Babylon the great" (Rev 16:19).

The great city was split into three parts, and the cities of the nations fell. Babylon the great was remembered before God, to give her the cup of the wine of His fierce wrath. (Rev 16:19)

The apostle is first introduced to Babylon after the seventh trumpet by an angel flying in mid-heaven:

And another angel, a second one, followed, saying, "Fallen, fallen is Babylon the great, she who has made all the nations drink of the wine of the passion of her immorality." (Rev 14:8)

After the seventh bowl and last plague had been dispensed, an angel explains the meaning of Babylon.

Then one of the seven angels who had the seven bowls came and spoke with me, saying, "Come here, I will show you the judgment of the great harlot who sits on many waters, with whom the kings of the earth committed acts of immorality, and those who dwell on the earth were made drunk with the wine of her immorality." (Rev 17:1-2)

The apostle soon learns that Babylon is an entity that deceives and removes any who attempts to teach the truth of mankind's sinful state and need of a Savior. And mankind is no less guilty for enjoying and reveling in this immoral worldview and lifestyle (Rev 17:1-18; 18:1-24).

The seventh bowl is significant for being the last of God's wrath. From this moment on, God will no longer have a reason for anger, because He has pronounced judgment on the last of sin. This final judgment is celebrated in heaven.

After these things I heard something like a loud voice of a great multitude in heaven, saying, "Hallelujah! Salvation and glory and power belong to our God; because His judgments are true and righteous; for He has judged the great harlot who was corrupting the earth with her immorality, and He has avenged the blood of His bond-servants on her." And a second time they said, "Hallelujah! Her smoke rises up forever and ever." (Rev 19:1-3)

And the twenty-four elders and the four living creatures fell down and worshiped God who sits on the throne saying, "Amen. Hallelujah!" (Rev 19:4)

And a voice came from the throne, saying, "Give praise to our God, all you His bond-servants, you who fear Him, the small and the great." (Rev 19:5)

Then I heard something like the voice of a great multitude and like the sound of many waters and like the sound of mighty peals of thunder, saying, "Hallelujah! For the Lord our God, the Almighty, reigns." (Rev 19:6)

With judgment passed, all that remains is Jesus Christ executing that sentence upon all those human beings who are left.