It is important because it is the truth.
Yet the assertion is completely unsubstantiated. Rather, we have plenty of passages that say that Mary had children who were Jesus' brothers and sisters.
The Bible does not contain a direct evidence that Mary had other biological children, because the word “brother” is often referring to relative not of the same mother.
Then you should be easily able to provide an example of where this is so in the NT. Merely declaring that "brother" always means "cousin" is the argument of fools. And we aren't fools, so try a little harder to prove that Mary was indeed a sinner in denying her husband the foundational commands of God "The two shall cleave together" and "be fruitful and multiply". Because that is what you are saying, that Mary willfully violated the first and foundational commands of God.
This Mary idolatry is getting out of hand. She indeed gave birth to the Messiah, and she did indeed enjoy her husband and bore him, in obedience to God, many children.
AMEN
Not "always". In a large family, when the outsider would not know or would not care who exactly is a biological brother, adopted brother, half-brother, step-brother, cousin, the generic term is "brother". The examples of that are Lot and Abram eing "brothers". In the New Testament we see the same usage: in Mark 6:3 Jesus is called brother of James, and Joseph, and Jude, and Simon, but in Mark 15:40 it is another Mary who is named as the mother of the fisrt two.
This does not prove that Jesus had no biological siblings -- the scripture does not contain such proof -- but it proves that the teaching that Mary the Mother of God remained virgin, which the Church held based on the historical knowledge outside the scripture -- does not contradict the scripture.