Friday after Ash Wednesday: “Why do we fast?"

The days will come when the bridegroom is taken away from them,
and then they will fast.”

-- Mt 9:14-15

The fourth precept ("You shall observe the days of fasting and abstinence established by the Church") ensures the times of ascesis and penance which prepare us for the liturgical feasts and help us acquire mastery over our instincts and freedom of heart.

-- CCC 2043

The New Law practices the acts of religion: almsgiving, prayer and fasting, directing them to the "Father who sees in secret," in contrast with the desire to "be seen by men." Its prayer is the Our Father.

-- CCC 1969