So here are a few reasons why so many have viewed Scripture memorization as so essential to the Christian life.
Paul wrote that “we all, . . . beholding the glory of the Lord, are being transformed into the same image from one degree of glory to another” (2 Corinthians 3:18). If we would be changed into Christlikeness, we must steadily see him. This happens in the word. “The Lord revealed himself to Samuel at Shiloh by the word of the Lord” (1 Samuel 3:21). Bible memorization has the effect of making our gaze on Jesus steadier and clearer.
“How can a young man keep his way pure? By guarding it according to your word. . . . I have stored up your word in my heart, that I might not sin against you” (Psalm 119:9, 11). Paul said that we must “by the Spirit . . . put to death the [sinful] deeds of the body” (Romans 8:13). The one piece of armor used to kill is the “sword of the Spirit” which is the word of God (Ephesians 6:17). As sin lures the body into sinful action, we call to mind a Christ-revealing word of Scripture and slay the temptation with the superior worth and beauty of Christ over what sin offers.
When Jesus was tempted by Satan in the wilderness, he recited Scripture from memory and put Satan to flight (Matthew 4:1–11).
The times when people need you to give them comfort and counsel do not always coincide with the times you have your Bible handy. Not only that, the very word of God spoken spontaneously from your heart has unusual power.Proverbs 25:11 says, “A word fitly spoken is like apples of gold in a setting of silver.” That is a beautiful way of saying, When the heart full of God’s love can draw on the mind full of God’s word, timely blessings flow from the mouth.***the above is a direct quote from http://www.desiringgod.org/articles/why-memorize-scripture ***