During the antediluvian period, I believe the purpose was to corrupt all of humankind so as to disqualify the promised Messiah's claim to having sinless blood.
Whether it's still possible to hybridize people after the flood, I don't know. I do understand that we underwent some pretty dramatic biological changes, the radically shorter lifespans being an indicator, and that the earth itself is fundamentally different. (Remember, Noah had never seen a rainbow? Apparently there was no direct, undiffused light from the sun- perhaps an effect of the mist that watered the earth because there was also no rain?) I also believe that we don't even approach the full comprehension of what really happened during the flood. I think that the remaining large dinosaurs were wiped out because they were biologically unable to handle the atmospheric changes.
But you believe that this human interbreeding continues to this day. Why? Can you reference that so I can check it out? I would think that cultures such as voodoo in Haiti or primitive animists in Papua New Guinea would have achieved this, given their open and close contact with demonic powers. (I understand the Canaanites were after the flood, but I take that as a result of Noah's curse over Canaan after Ham dishonored Noah- therefore, continuing the physical pre-flood condition supernaturally.)
King David killed Goliath--a product of such ancestry. It, sadly, didn't stop with the flood.
Most of the better UFO sites--such as ABOVE TOP SECRET have plenty of docs about the breeding aspects of abductions.
There are countless personal experiences of women being abducted, impregnated in horrid UFO sorts of settings with alien 'intrusions' etc. . . . or being impregnated with a fertilized egg evidently the egg from an ET and an unwilling sperm donor . . . and before the first trimester is over, the woman goes to bed pregnant--and wakes up after another abduction not pregnant.
Many report being shown their hybrid children at varying stages of development--all faster than normal total humans.
Search ATS and you should be able to come up with the common scenarios re such.