Albert Barnes Commentary Deuteronomy 21:12

"then thou shalt bring her home to thy house; and she shall shave her head, and pare her nails;" — Deuteronomy 21:12 (ASV)

Shaving the head (a customary sign of purification, Leviticus 14:8; Numbers 8:7), and the putting away of the garment of her captivity, were designed to signify the translation of the woman from the state of a pagan and a slave to that of a wife among the covenant-people. Consistency required that she should pare (dress, compare 2 Samuel 19:24), not suffer to grow, her nails; and thus, as far as possible, lay aside everything belonging to her condition as an alien.