IMO, acknowledging the reality humans other than those created in the image of God existed alongside Adam and his descendants, answers a great many questions about the creation story of Genesis that can not be rationalized any other way. Even though the Scripture says Adam and Eve had more sons and daughters, other than the three sons who are named in the Genesis narrative.. The question remains, "Who did Cain marry" whom God banished and cursed ?! He couldn't have married any of Adam's descendants, now could he ?!
There is also the obvious question and problem of who Seth married ? Did he marry one of his own sister ?! Forget about the later biblical prohibition against incest, what about the problem of genetic diversity ?! One couple, a man and a woman, can not constitute a viable breeding population of humans. It's scientifically impossible. Interbreeding between Adam's descendants would have eventually led to a dead end. Adam's offspring would have succumb to genetic abnormality, infant mortality and congenital disease.
In a few more years, there won't be any more tigers in captivity. The captive tiger population has been interbred to the point, the remaining tigers are no longer viable candidates for reproduction. The same outcome would result in a small and isolated population of humans that interbred. There had to be a diverse and unrelated human population available in order for mankind to be "fruitful, to multiple and fill the earth".
Theologians have come up with all kinds of extraneous and erroneous theories to try and answer the question. Everything from proposing God suspended natural law with some miracle, to Adam's descendants having sex with angels. IMO, none of which is explicitly described anywhere in the Genesis narrative.
The existence of non-adamic man, also explains the mystery in the story of the Biblical Flood, where the patriarch Noah (after having followed God's instructions to the letter, and survived the flood), then becomes a drunk and lays naked before God. What is that all about ??? Obviously there was a problem between Noah and God. The problem isn't specifically addressed in the narrative, but it is alluded to in the flood story itself.
What did Noah think when he released a dove and it returned with an "olive twig plucked off a tree" in its' beak ?! Noah would have realized instantly, that twig didn't come off any tree that had been under the waters of the flood. He would have also realized intuitively the flood could not have been a global event, but could only have been local in nature.. and somewhere out there, the descendants of Cain had also survived the flood.
In Noah's mind, the flood and its destruction, had been totally without meaning, so he plants a vineyard and becomes a drunk. Mystery solved.
What Noah didn't understand, was that God had correctly destroyed the segment of humanity that was the problem. It was the descendants of Adam and Cain that had filled the earth with violence and needed to be destroyed... not primordial man. There is no archeological evidence to substantiate any claim homo-sapiens were warlike, or violent creatures. No skeletal remains have ever been found showing evidence of homo-sapiens murdering each other with weapons. It simply doesn't exist.
Homo sapien-sapien (modern man) however is a different story. Our history is a litany of continuous and ongoing violence from the time Cain slew Able, right up until modern times. Now we have developed the weapons to destroy the Earth itself and all the creations therein.. Which will constitute a violation of Gods everlasting covenant with Noah (The Noahic Covenant), in which God promises never again to destroy the earth because of the wickedness of mankind.