Here is the reason given by Charles H. Spurgeon:
"I am persuaded that the use of a good Catechism in all our families will be a great safeguard against the increasing errors of the times,... Those who use it [a good Catechism] in their families or classes must labour to explain the sense; but the words should be carefully learned by heart, for they will be understood better as years pass. May the Lord bless my dear friends and their families evermore, is the prayer of their loving Pastor. C. H. Spurgeon."
The Greek word for "instruct" or "teach" is katecheo. This is where we get the word "catechize." It simply is teaching biblical truth in an orderly way. Most often catechism is a series of questions and answers along with biblical evidence (Bible verses) and explanation.
4 “Hear, O Israel: The Lord our God, the Lord is one. 5 You shall love the Lord your God with all your heart and with all your soul and with all your might. 6 And these words that I command you today shall be on your heart. 7 You
shall teach them diligently to your children, and shall talk of them
when you sit in your house, and when you walk by the way, and when you
lie down, and when you rise. 8 You shall bind them as a sign on your hand, and they shall be as frontlets between your eyes. 9 You shall write them on the doorposts of your house and on your gates.