The Immaculate Conception doesn't refer to the conception and birth of Jesus. It refers to the conception of Mary free from all sin in her mother's womb. This special (and unmerited!) favor was accorded her by God in anticipation of her role as the living ark for the living Word of God. She didn't, strictly speaking, HAVE to have this done for her, but "it was fitting and proper," as the eastern Church says. She needed the salvation of the cross just like everyone else, but the fruit of the cross was applied to her beforehand.
I appreciate your explanation, but could you kindly cite any Scripture to support your assertion?