There is a need for it.

Discernment is the ability to judge well, to distinguish between right and wrong, good and evil. It is the ability to see things as they really are, to see the truth. It is the ability to see things from God's perspective. It is the ability to see the world as God sees it. Jesus calls us to have discernement in our lives. This

Matthew 7:15

Beware of false prophets, who come to you in sheep's clothing but inwardly are ravenous wolves.

Why are we called to exercise discernment?

Simply put, so that we may not be lead into sin. There is a saying that goes: "We are the sum of the five people around us". Normalizing sin leads to a poisoining of the soul and heart. It is repeated in both the old and new testament.

Proverbs 13:20

Whoever walks with the wise becomes wise, but the companion of fools will suffer harm.

1 Corinthians 15:33

Do not be misled: “Bad company corrupts good character.”

How to discerne?

Through the Holy Spirit and the Word of God. The Scriptures are clear about the qualities and behaviors that are righteous in the eyes of God. A beautiful verse that summarizes those for us in one concept, the fruit of the Spirit.

Galations 5:22-23

But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law.

Let us then apply these biblical characteristics to the people surrounding us. You might realize that X or Y person does not meet any of these criteria, they do not bear the fruit of the Spirit.
Now, this is not a call to codemn, but a call to help and exhort them to follow the righteous path. In the best case you win your brother or sister back to Christ. In the worst case, you understand the need to distance yourself from them. There is such a thing as righteous judgement.

The grace of the Lord Jesus be with us all. Amen.