Why.

Matthew 5:31
It has also been said, 'Whoever divorces his wife must give her a certificate of divorce.'

Deuteronomy 24:1-4
If a man marries a woman, but she becomes displeasing to him because he finds some indecency in her, he may write her a certificate of divorce, hand it to her, and send her away from his house. . . .

Isaiah 50:1
This is what the LORD says: "Where is your mother's certificate of divorce with which I sent her away? Or to which of My creditors did I sell you? Look, you were sold for your iniquities, and for your transgressions your mother was sent away.

Jeremiah 3:8
Because faithless Israel had committed adultery, I gave her a certificate of divorce and sent her away. Yet I saw that her unfaithful sister Judah had no fear and prostituted herself as well.

Mark 10:4
They answered, "Moses permitted a man to write his wife a certificate of divorce and send her away."

and to.

Matthew 1:19
Because Joseph her husband, a righteous man, was unwilling to disgrace her publicly, he resolved to divorce her quietly.

Malachi 2:16
"For I hate divorce, " says the LORD, the God of Israel, "and I hate when a man covers himself and his garment with violence, " says the LORD of Hosts. Therefore guard yourselves in your spirit and do not break faith.