Why must a Yoledet, a woman who just gave birth, bring a sin offering?

Why must a יולדת bring a sin offering? What does she need כפרה for?

As it says in Vayikra 12:6-7:

וּבִמְלֹאת יְמֵי טָהֳרָהּ לְבֵן אוֹ לְבַת תָּבִיא כֶּבֶשׂ בֶּן שְׁנָתוֹ לְעֹלָה וּבֶן יוֹנָה אוֹ תֹר לְחַטָּאת אֶל פֶּתַח אֹהֶל מוֹעֵד אֶל הַכֹּהֵן

6 And when the days of her purification have been completed, whether for a son or for a daughter, she shall bring a sheep in its first year as a burnt offering, and a young dove or a turtle dove as a sin offering, to the entrance of the Tent of Meeting, to the kohen.

וְהִקְרִיבוֹ לִפְנֵי יְהֹוָה וְכִפֶּר עָלֶיהָ וְטָהֲרָה מִמְּקֹר דָּמֶיהָ זֹאת תּוֹרַת הַיֹּלֶדֶת לַזָּכָר אוֹ לַנְּקֵבָה

7 And he shall offer it up before the Lord and effect atonement for her, and thus, she will be purified from the source of her blood. This is the law of a woman who gives birth to a male or to a female.

If you quote a commentary please explain what it means if you can. Thanks!