All of the Bible is important, but some parts are more critical than others for Christian living. For example, you could memorize the 12 tribes of Israel, but you might find it more useful to spend your time meditating on other portions of Scripture, ones that focus on how to follow Jesus.
Chance are good that you have never said to yourself as you were trying to fight off temptation, “I sure am glad I memorized the names of the 12 apostles!”
This is not to say that such endeavors are not worthy of our time and effort. However, we might do better to concentrate on other parts of the Bible—parts that build our character and help us to live holy lives. This is the reason we learn what the Bible says.