Why the virgin birth; was Mary of Davidic line; did her sinlessness matter?

Mary descended directly from Abraham and David, fulfilling the promise made to David’s line. She was the virgin prophesied in Isaiah who would bear Emmanuel, “God with us.” She was not only a virgin but immaculate, without original or actual sin. Her virginity prevented any transmission of sin through human seed. Her sinlessness ensured the humanity of Christ was pure. That is why both her perpetual virginity and her immaculate conception are necessary truths.