Or HaChaim on Leviticus 12:7/אור החיים על ויקרא י״ב:ז׳

1 והקריבו לפני ה׳ וכפר עליה; and he shall offer it before G'd and make atonement for her. Concerning the offering of the sheep as a burnt-offering the Torah said והקריבו, whereas concerning the turtle-dove as the sin-offering the Torah writes וכפר עליה, "and he will make atonement for her." Torat Kohanim write that the two sacrifices are not both dependent on each other in order to fulfil their respective tasks but that only one depends on the other. I do not know therefore which of the two sacrifices is indispensable, and this is why the Torah said והקריבו, he will offer it up. Had, the Torah only written these words I still would not have known which one of these two sacrifices was indispensable; therefore the Torah adds the words וכפר עליה to tell us that the sin-offering is indispensable. Perhaps the exegesis is derived from the fact that the Torah could have simply written והקריבם, "and he is to offer them up." The fact that the Torah spent all these extra words teaches that the two sacrifices are not of equally indispensable nature.