I don't know if God thought it was necessary, but He certainly decided it was fitting that Mary be immaculate, preserved from original sin.
Remember, the teaching is NOT that Mary didn't need a Savior. The teaching is that she was freed BEFOREHAND from sin and saved, in anticipation of the merits of her Son on the Cross.
That was the Franciscan opinion, I believe, for centuries. The question on how she could have been saved before the Redemption had been argued for a long time.
We were saved AFTER we had fallen into the pit (of original sin). Mary was saved BEFORE she could fall in, by a singular grace and privilege of Almighty God.
But the idea is a total fabrication....There is nothing to base the fable on...And it goes against the word of God in the scriptures...