Why do catechism?
Click here for a really helpful article from Tim Keller about this. I've quoted part of it below, where he answers the question: "Is catechism a biblical practice?"
"In his letter to the Galatians Paul writes, "Anyone who receives instruction in the word must share all good things with his instructor" (Galatians 6:6). The Greek word for "anyone who receives instruction" is the word katechoumenos, one who is catechized. In other words, Paul is talking about a body of Christian doctrine ("catechism") that was taught to them by an instructor (here the word "catechizer"). The words "all good things" probably means financial support as well. In this light, the word koinoneo—which means "to share" or "to have fellowship"—becomes even richer. The salary of a Christian teacher is not to be seen simply as a payment but a "fellowship." Catechesis is not just one more service to be paid for, but is a rich fellowship and mutual sharing of the gifts of God."
"If we re-engage in this biblical practice in our churches, we will find again God's Word "dwelling in us richly" (Colossians 3:16), because the practice of catechesis takes truth deep into our hearts, so we find ourselves thinking in biblical categories as soon as we can reason."