timdiggerm said:

Why is the immaculate conception a necessity? Why does Mary have to be without sin in order to give birth to Christ? How could she be born without sin?

We believe that Mary was predetermined to be the mother of God. If you put drinking water in a pitcher, you want that pitcher to be completely clean. We believe that it was the same with Mary. She was going to be a "vessel" through which God would become incarnate and come into the world as a human being, so God made her without original sin so that she would be as pure and holy a vessel as possible. Technically she didn't have to be without sin in order to give birth to Christ, but you could say that for a pitcher of water. Technically you can drink the water without cleaning the dirty pitcher, but most people (in the first world anyway) would clean it first. She was preserved from sin so that she would be prepared for carrying Christ in her womb, just as you might clean your house and try to keep your it as clean as possible when you have important guests coming over.