To recap the lesson Genesis 1 through 4: any similarity between these stories and the actual events of Creation is purely coincidental.
Chapter 5 recounts the mythical patrilineal genealogy from Adam to Noah. Fabulous genealogies are not uncommon, and they may serve to bind disparate clans into tribes and tribes into nations. The exceedingly long lives of the progenitors of Noah and Noah himself formed part of the basis for the famous calculation of Bishop Ussher that Creation took place in 4004 BC. We have no evidence outside this story to corroborate the claim that men once lived for up to a millenium, and I don't suppose that it signifies anything in any event. We might just as easily have skipped this and gone straight to Noah with a segue such as "And it came to pass many generations later..." It appears that this genealogy represents the line of the eldest sons (excluding Cain) of the eldest sons, and it is odd that these men sometimes waited centuries to father children.
Chapter 6 is chock full of tastes of what might have once been a more comprehensive mythology. Here we find the "sons of God" mating with human women and begetting mighty men. Also, God sets the human lifespan at 120 years, a number which is close to the human maximum life span to this day. Here also we find God regretting the Creation of every living thing and contemplating the destruction of life. He relents in this plan and instead decides to stary over with Noah and his family and an ark full of every living thing. Everyone and everything else is destined to be wiped out in a flood of a planetary scale.
In Chapter 7, God makes good on the threat to annihilate every land animal except the ones in the ark.
In Chapter 8, Noah waits for the flood waters to recede. God promises never to wipe everything out again.
Aside from this story, and similar myths about a deluge from that region, we have no evidence that any such global flood occurred or that mankind was reduced to a population of eight in the recent past. In fact, the entire story is utterly preposterous. Did Noah make it to Australia to gather up the marsupials and monotremes and then return them to their habitats? That would make a good story. Wouldn't the platypus have been mentioned somewhere? There was an apparent population bottleneck about 70,000 years ago which may have reduced the human population to about 10,000 souls, but I doubt that this was the inspiration for the story of Noah and his ark.
What do we gain from studying this myth? The idea that God might not have known what He was doing when he created life and that he was capable of regret. This is a far cry from the omniscient God we conceive of today who would have known that mankind would turn out the way he did and who would not have had to call a mulligan. The story is most interesting for what it says about the ancient Hebrew conception of God. Their God was more anthropomorphic and had a much smaller portfolio than our God today. Perhaps we should keep this in mind when we read the rest of the Old Testament. The concept of God has clearly evolved, notwithstanding the claim that He is the same Yesterday, Today and Tomorrow. Perhaps God is the same, but human understanding of Him is obviously not.
Other than the lesson that the ancient Hebrews had some very different ideas about God, a lesson which will doubtless be lost on many readers, I don't reckon this story signifies at all. We could have very well gotten along without it just fine with no adverse impact on theology. As far as I'm concerned, we could just start at Genesis 9 and be none the worse off. We may even be better off because the cognitive dissonance of considering a loving God who kills almost every human on earth could be avoided.