Jamieson, Fausset and Brown on Genesis 7:2-3

2-3. Of every clean beast ... fowls--Pairs of every species of animals, except the tenants of the deep, were to be taken for the preservation of their respective kinds. This was the general rule of admission, only with regard to those animals which are styled "clean," three pairs were to be taken, whether of beasts or birds; and the reason was that their rapid multiplication was a matter of the highest importance, when the earth should be renovated, for their utility either as articles of food or as employed in the service of man. But what was the use of the seventh? It was manifestly reserved for sacrifice; and so that both during Noah's residence in the ark, and after his return to dry land, provision was made for celebrating the rites of worship according to the religion of fallen man. He did not, like many, leave religion behind. He provided for it during his protracted voyage.