Pope Francis goes to Confession at St. Peter's Basilica on March 28, 2014. (Credit: © L'Osservatore Romano)
The next time that you are questioned about “why you confess to a man,” this article might be a handy aid.
One technique or method I have come up with in the course of my apologetics, is to answer questions about theology with Bible passages only, in order to demonstrate how intensely “biblical” Catholic teaching actually is.
Regarding this particular question, Catholics are often accused of “inventing” confession and absolution alike (i.e., aspects of the sacrament of penance). In fact, no parts of Catholic belief in this area are without explicit biblical sanction or model. One just has to do a little Bible study and “look around” a bit.
I will use the RSV version.
1. The Church Dispenses Forgiveness (Sacrament of Penance)
Was the forgiveness of sins through priests foreshadowed in the old covenant?
Leviticus 5:5-6 When a man is guilty in any of these, he shall confess the sin he has committed,  and he shall bring his guilt offering to the LORD for the sin which he has committed, a female from the flock, a lamb or a goat, for a sin offering; and the priest shall make atonement for him for his sin.
Leviticus 19:21-22 but he shall bring a guilt offering for himself to the LORD, to the door of the tent of meeting, a ram for a guilt offering.  And the priest shall make atonement for him with the ram of the guilt offering before the LORD for his sin which he has committed; and the sin which he has committed shall be forgiven him.
Did prophets ever function as God’s intermediaries in the area of forgiveness?
2 Samuel 12:13 David said to Nathan, “I have sinned against the LORD.” And Nathan said to David, “The LORD also has put away your sin; you shall not die.”
Is there any passage showing that a priest grants absolution?
Matthew 18:18b … whatever you loose on earth shall be loosed in heaven.
Does a priest have the authority to forgive sins as God’s representative?
John 20:23a If you forgive the sins of any, they are forgiven;…(cf. 2 Cor 2:10; see below)
Is confession a biblical concept?
Psalm 38:18 I confess my iniquity, I am sorry for my sin.
Proverbs 28:13 He who conceals his transgressions will not prosper, but he who confesses and forsakes them will obtain mercy.
Acts 19:18 Many also of those who were now believers came, confessing and divulging their practices
Is confession of sins to a spiritual authority specifically mentioned?
Mark 1:4-5 John the baptizer appeared in the wilderness, preaching a baptism of repentance for the forgiveness of sins.  And there went out to him all the country of Judea, and all the people of Jerusalem; and they were baptized by him in the river Jordan, confessing their sins. (cf. Mt 3:6)
James 5:14-16 Is any among you sick? Let him call for the elders of the church, and let them pray over him, anointing him with oil in the name of the Lord;  and the prayer of faith will save the sick man, and the Lord will raise him up; and if he has committed sins, he will be forgiven.  Therefore confess your sins to one another, and pray for one another, that you may be healed. The prayer of a righteous man has great power in its effects. (cf. 1 Jn 1:9)
2. Authority of Priests to Impose Penance
Do priests have the authority to assign penance to sinners?
Matthew 18:18a Truly, I say to you, whatever you bind on earth shall be bound in heaven, . . .
Do priests have the authority to “retain” sins?
John 20:23b …if you retain the sins of any, they are retained.
Is penance after confession of sins seen in the old covenant?
Numbers 5:7 he shall confess his sin which he has committed; and he shall make full restitution for his wrong, adding a fifth to it, and giving it to him to whom he did the wrong.
Did St. Paul assign penance to an unrepentant sinner?
1 Corinthians 5:1-5 It is actually reported that there is immorality among you, and of a kind that is not found even among pagans; for a man is living with his father's wife.  And you are arrogant! Ought you not rather to mourn? Let him who has done this be removed from among you.  For though absent in body I am present in spirit, and as if present, I have already pronounced judgment  in the name of the Lord Jesus on the man who has done such a thing. When you are assembled, and my spirit is present, with the power of our Lord Jesus,  you are to deliver this man to Satan for the destruction of the flesh, that his spirit may be saved in the day of the Lord Jesus.
Did Paul even publicly name sinners to whom he assigned penance?
1 Timothy 1:19-20 holding faith and a good conscience. By rejecting conscience, certain persons have made shipwreck of their faith,  among them Hymenae'us and Alexander, whom I have delivered to Satan that they may learn not to blaspheme.
3. Indulgences (Relaxation of Temporal Punishment)
Does the Church have the authority to grant an indulgence?
2 Corinthians 2:6-11 For such a one this punishment by the majority is enough;  so you should rather turn to forgive and comfort him, or he may be overwhelmed by excessive sorrow.  So I beg you to reaffirm your love for him.  For this is why I wrote, that I might test you and know whether you are obedient in everything.  Any one whom you forgive, I also forgive. What I have forgiven, if I have forgiven anything, has been for your sake in the presence of Christ,  to keep Satan from gaining the advantage over us; for we are not ignorant of his designs.
The next time that you are questioned about “why do you confess to a man?” and suchlike, perhaps this article might be a handy aid.