John Calvin Commentary Genesis 7:13

"In the selfsame day entered Noah, and Shem, and Ham, and Japheth, the sons of Noah, and Noah`s wife, and the three wives of his sons with them, into the ark;" — Genesis 7:13 (ASV)

In the self-same day entered Noah, and Shem, etc. A repetition follows, sufficiently detailed, considering the brevity with which Moses recounts the history of the deluge, yet by no means superfluous. For it was the design of the Spirit to keep our minds fixed on a vengeance too terrible to be adequately described by even the most severe language.

Besides, nothing is related here except what is difficult to believe; therefore Moses more frequently emphasizes these things, so that however remote they may be from our understanding, they may still be believed by us. Thus the narrative concerning the animals relates to this point: that by the faith of holy Noah they were drawn from their woods and caverns and were collected in one place from their wandering paths, as if they had been led by the hand of God.

We see, therefore, that Moses does not insist upon this point without a purpose; but he does it to teach us that each species of animals was preserved, not by chance, nor by human industry, but because the Lord reached out and offered to Noah himself, from hand to hand (as they say), whatever animal He intended to keep alive.