How is the reiteration of “Be fruitful, and multiply” (Gen 9:1) significant?

It serves two functions. First, it underscores the notion that Noah and his family are blessed by presiding over a new Earth, as it were a new creation; it was after the introduction of man to the first creation that God gave him the same blessing. Notice this serves not just as a command but as a blessing, the profundity of which becomes only clearer between this verse and others concerning the sanctity of life, leading up to Gen 9:7 where “be fruitful, and multiply” is again reiterated. The second function is to make a general statement of approval of the notion that human life on earth is to be replenished, even though God has just stated grimly that “the imagination of man’s heart is evil from his youth.” (Gen 8:21)