Good question. May I humbly recommend my book Mary: A Catholic-Evangelical Debate? Even for those who are not Evangelicals, the Catholic arguments are put best I can.
One answer to the question is this: to be fully involved in God’s plan of redemption, Mary had to make a totally free choice whether to obey God’s invitation or not. The problem is, that original sin tilts our free will in the direction of disobedience. If Mary had not been free of original sin, she would not have had a totally free choice. Her choice would have been biased toward saying ‘No’. In order to have a free choice (like Eve’s) she had to be delivered from original sin. This freedom from original sin did not bias her toward saying ‘yes’ or guarantee that she would say, ‘Yes’. It simply put her in a totally free condition where there was no bias either way. From this unbiased position she was totally free to co-operate with God or not.
Once this necessity is seen it then remains to ask, “If this is so, how was she preserved from original sin, and when did this state of perfection begin?” The church has answered this by explaining that she was preserved from the stain of original sin by a special act of grace (merited by her Son’s redemptive death). When did this begin? It must have begun when her life began. When did her life begin? At conception.