Looking for why it is being told 'the big message'

Genesis 6:1-22
The Lord said , "My Spirit shall not abide in man forever, for he is flesh; his days shell be 120 years."
The wickedness of man was great in the earth, and that every intention of the thoughts of his hearts was only evil continually. And the Lord was sorry that he had made man on the earth.
"Behold, I will bring a flood of waters upon the earth to destroy all flesh in which is the breath of life under heaven. Everything that is on the earth shall die. But I will establish my covenant with you, and your family." God said to Noah.
Different versions give 'sons of the gods' of 'sons of God' or 'supernatural beings.'
Why?
The human race, lost in violence and corruption, is bringing destruction upon itself. God reduces the life-span to 120 years. But that alone will not answer. Only an act of judgment will rid the world of sin.
As with creation, we should approach this account as story, expecting picture and symbol and pattern, looking for why it is being told, 'the big message.'
When people's every intention of the thoughts of his hearts are only evil, destruction will come upon him for their wickedness. But God gives opportunity to everyone to repent and turn from evil.
How?
Looking for why it is being told 'the big message'.