It was the purpose of Satan and his fallen angels to corrupt the human race and thereby do away with pure Adamite stock through whom the seed of the woman should come. This would avert their own doom and make it possible for Satan and his kingdom to keep control of the planet earth indefinitely. It was said to Adam and Eve that the seed of the woman should defeat Satan and restore man’s dominion [Gen. 3:15]. The only way then, for Satan to avoid this predicted defeat was to corrupt the pure Adamite line so that the coming of the seed of the woman into the world would be made impossible. This, he tried to accomplish by sending some of his fallen angels to marry the daughters of men as in Gen. 6:1-4, and producing the giant nations through them.
There are two such eruptions of fallen angels taught in Gen. 6: 4: There were giants in the earth “in those days” [before the flood]]: and “also after that” [after the flood], when the sons of God [fallen angels] came in unto the daughters of men [any daughters of men – Cain, Seth and others], and they bare children to them [to the angels].
Satan almost succeeded in his plan during the first eruption, for “all flesh had corrupted his way upon the earth:” and of all the multitudes Noah and his sons were the only pure Adamites left to be preserved by the ark [Gen. 6:8-13; 1 Pet. 3:19-20]. The main object of the flood was to do away with all this satanic corruption, destroy the giants, and preserve the pure Adamaite stock so as to make good the guarantee of the coming of the seed of the woman, as in the plan of God.
Being defeated before the flood did not stop Satan from making a further attempt to prevent the coming of the Redeemer who should be his final downfall. It is now to his advantage that God had promised never to send another universal flood upon the earth. Satan therefore reasoned that he should make a second attempt to do away with Adamite stock. If he came within “eight souls” of doing it before the flood his opportunities were now ever greater, with the promise that there would be no such flood. This is the reason for the second group of his fallen angels being sent to marry the daughters of men. Once again the unions produced giants, and races of them occupied the land of promise, where the seed should be born, in advance of Abraham. Limited by His promise of no flood, God was then faced with the problem of destroying the giant races another way.
This explains why He commanded Israel to kill them everyone, even to the last man, woman and child. This again explains why He destroyed all the men, women and children besides Noah and his family, at the time of the flood. It also answers the skeptic question regarding why the children were taken away with the adults in the flood. God had to do away with this corruption entirely in order to fulfill His eternal plan and give the world its promised Redeemer. The Redeemer has come now, and so Satan is reserving his forces for a last stand at the 2nd coming of Christ.
Thus it is clear from Scripture that there were giants in the earth both BEFORE AND AFTER the flood and that they came from a union of fallen angels and the daughters of men.
– Dake Annotated Reference Bible: page 63