Even though we cannot be sure why that phrase is repeated, we can certainly deduce that it's inclusion serves to illustrate that the World was being rapidly filled. This may have also been used to contrast the large populous in contrast to there being only Noah found worthy of escaping the flood.
Genesis 6:5 through 13 KJV And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. And it repented the LORD that he had made man on the earth, and it grieved him at his heart. And the LORD said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them. But Noah found grace in the eyes of the LORD. These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God. And Noah begat three sons, Shem, Ham, and Japheth. The earth also was corrupt before God, and the earth was filled with violence. And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth. And God said unto Noah, The end of all flesh is come before me; for the earth is filled with violence through them; and, behold, I will destroy them with the earth.
It is also worthy to note that only Noah found Grace; and yet God also saved his three sons and their wives. Later in Genesis we find that through these three sons both good and evil, were continued in the World.
Why would God continue both in the World? I cannot say for sure, but it would seem to me that unless both were preserved there would remain no freedom of choice.