God wanted Mary pure and immaculate

By her immaculate conception, the Virgin Mary was preserved from original sin in anticipation of her divine motherhood and the future merits of her son Jesus. This means that she is the first and the most perfectly redeemed of all creatures, and that the salvation won by her Son also applies to her. …

The early redemption of the Virgin and her preservation from sin by pure grace does not mean that her conduct was not heroic or exemplary: she did remain free. In other words, she wasn’t just spotless in her inmost being, but also in her actions. She consented freely to her own exemption from sin. She had to practice virtue every day.

When we invoke her as the Immaculate Conception, we are not only praising her predestination by God to be all pure and immaculate. We also thank her for her actions on our behalf to protect us from the devil's snares. Mary's conduct was meritorious in the sense that her clear perceptiveness was coupled with her firm resolve to remain free of sin towards the salvation of mankind. She is completely deserving of our veneration, and it is sheer justice to acknowledge her immense perseverance and suffering love.

Mary was not conceived without sin for her own benefit, but so that she could give birth to us, along with God, albeit on a different level—in the life of grace. She doesn’t keep her purity jealously to herself or revel in her own perfection (this absence of complacency being the sign that sin had no power over her), but she puts her qualities to the service of the mission that she received from the Trinity: to help us in our fight against evil. To this end, as our mother in the order of grace, she kept herself pure from any compromise with sin. 

Jean-Michel Castaing

Adapted from: Aleteia