Noach was a righteous man, perfect in his generation; Noach walked with G-d (Gen. 6:9)
The Torah goes into such detail to describe the righteousness of Noach to show that he was meticulous in observing both categories of mitzvot, those that involve serving G-d, and those that involve our responsibility to our fellow man.
"Perfect in his generation" refers to the proper way that Noach treated every human being, and "with G-d Noach walked" refers to the fact that Noach served G-d diligently.
(Pardes Yosef)
Noach was a righteous man, perfect in his generation; Noach walked with G-d (Gen. 6:9)
A righteous man, a tzaddik, is a person who "walks with G-d," so the above verse appears to be redundant. The Torah is showing us just how great a person Noach was.
He was indeed, "perfect in his generation," acting in a righteous manner when he was out among the people of his generation. Yet even when "he walked with G-d," alone, with only G-d to witness his actions, he still behaved in a righteous manner.
(Yalkut Hadrush)
All flesh has corrupted his way on the earth (Gen. 6:12)
In the days before the flood, the moral situation had deteriorated to the point that even those who by their nature recognized the difference between right and wrong lost that sensitivity and began to sin without feeling a sense of guilt and wrongdoing.
(Baal HaTurim)
G-d said to Noach, "Enter, you and all your family, into the ark." (Gen. 7:1)
Every detail in the Torah contains eternal lessons that we can utilize even in our times. The Hebrew word for "ark," teiva, also means "word." G-d is commanding every one of us to "enter" the words of Torah, to read each word with feeling and understanding.
(the Baal Shem Tov)