Could you please explain CC to me?
CC means holding down while standing on the ground. Crouch Cancel.
If you are at 0% and holding down you won't be knocked over by most of anything Marth throws at you.
Yes you do still take damage (same amount) but you can emediatly retaliate with say a grab which leads to a guaranteede knock down and so you come out on top.
Most people know all about this and the specific percentages and thresholds and so they won't go for "weak" attacks when the opponent is still at CC range.
You see many newer sheiks Dash Attack an experienced player at 0%. They simply crouch and grab the sheik.
Depending on what character you are up against you need to wait using DA until it lift them off the ground even if they hold down.
This is why needles are so important. And Sheiks grab game.