I don’t buy that she was sinless.
The doctrine of the Immaculate Conception does NOT hold that Mary was
unable to sin. It holds that She was conceived in the same state as Eve: free from original sin. And, unlike Eve, She said NO to the devil and therefore did not lose the original grace that Adam and Eve had been created with. Further, Mary did not commit any actual sin during Her life.
Now… here is why it matters.
At the time of the fall of man, God has to be 100% sure that the plan of salvation will go through: that Mary will say yes. Otherwise Christ doesn’t ransom us, and all of us are gonna go to hell. Now who doesn’t want it to be 100%? The devil.
Now, because of original sin, all of us are conceived with concupiscence (a weak, wounded conscience and tendency towards sin) and are to some extent under the devil’s power in a way that Adam and Eve were not before they fell. If Mary is not protected from that, the devil can exploit concupiscence the way he does with us. We fall all the time, don’t we? If Mary had fallen, ALL OF US DO, and the devil wins.
But wait, you may say… God can just find another Virgin to say yes. Infinite recursion. A train of just cabooses. No engine, goes nowhere. And so God would have to change the plan of salvation. He can’t do that. He made a promise in Genesis 3:15 and He has to keep it. God cannot change. (Malachi 3:6). He is Perfect, and change means lack; if you change, you lacked something before you changed. And if God could change one of His promises… what else can He change? We hold these truths to be self-evident, that all men are created equal and endowed by their Creator with… oh wait not anymore. Marriage? Abortion? Any question of morality… A changeable God is how you get today’s moral relativism.
The other “fix” is to deny free will: Mary is a robot and so are we. That’s Calvinism. Problem is, there is a hell with a population greater than zero. If we have no free will, God is the cause of our sins. Then the atheists are right: God is evil. Again, welcome to today’s dark world.
Now you see why Mary is a big deal, and why the devil will stop at nothing to keep you away from Her!
EDIT: split into new topic: No Immaculate Conception, No Immutable God