Act of Contrition [ from the rite of penance ]

Why do Catholics say an Act of Contrition?

Most commonly prayed at the end of confession, the Act of Contrition is a short Catholic prayer of penance and mercy. To express your “contrition” is to share your sorrow or remorse; the Act of Contrition is an apologetic prayer.

My God,
I am sorry for my sins with all my heart.
In choosing to do wrong
and failing to do good,
I have sinned against you
whom I should love above all things.
I firmly intend, with your help,
to do penance,
to sin no more,
and to avoid whatever leads me to sin.
Our Savior Jesus Christ suffered and died for us.
In his name, my God, have mercy.