Why do we say Amen?

This is in the Catholic catechism, but seems to be the same for all Christians.

"Why do we say "Amen" to the profession of our faith?

We say Amen—“Yes”—to the profession of our faith because God appoints us witnesses to the faith. Anyone who says Amen assents freely and gladly to God’s work in creation and redemption.

The Hebrew word amen comes from a family of words that mean both “faith” and “steadfastness, reliability, fidelity”. “He who says amen writes his signature” (St. Augustine). We can pronounce this unconditional Yes only because Jesus in his death and Resurrection has proved to be faithful and trustworthy for us. He himself is the human Yes to all God’s promises, just as he is also God’s definitive Yes to us."