Because Christ is God, it’s fitting that he took his humanity from a sinless human nature, although it wasn’t strictly necessary that his mother be sinless for him to receive from her a sinless human nature. God could have done it another way.
Nor was it absolutely necessary that Christ be born of a virgin. He could have come into the world via the normal route. The fact that Mary was a virgin and conceived Christ isn’t so much a statement about Mary as it is about the dignity of the child she carried in her womb.
Likewise (and ultimately) the Immaculate Conception isn’t so much a statement about the dignity of Mary as it is a statement about the dignity of her son. It points out who he is–God incarnate.