Perhaps the best reason for doing the Spiritual Exercises is that given by Our Lord Himself, and which St. Ignatius quotes in his book:
"What does it profit a man if he shall gain the whole world but loses his own soul?" (Mk 8:36)
It is very easy in this world to lose track of the meaning of our lives—why we were created and how we can find God. Many people have spent their lives working very hard to achieve success, only to find out at the end that they have wasted much of their time.
The goal of the Spiritual Exercises is refocus our lives on what is most important, first by correcting our sinful inclinations, and then by pointing us toward the ultimate goal of our lives, which is the glory and praise and service of God, our Creator and Redeemer.
The first step is to correct whatever in our lives is not ordered towards God. St. Ignatius refers to this as "preparing and disposing the soul to rid itself of all inordinate attachments..." For this reason the title of St. Ignatius's book of Spiritual Exercises reads: "Spiritual Exercises to conquer oneself and regulate one’s life without determining oneself through any tendency that is disordered."
What is the second step? St. Ignatius continues to say: "... and, after their removal [our sinful inclinations], of seeking and finding the Will of God in the disposition of our life for the salvation of our soul." An honest appraisal of our sinful tendencies is the precondition for the second step, which is discovering the Will of God. This should not be surprising because what most obscures our seeing the will of God is preoccupation with our own will.
This is the aim of the Spiritual Exercises: to first remove anything that might hinder us from seeking God's will for our lives, and then, once that's done, to seek and follow God's will. This is what the Spiritual Exercises are: a plan of conversion and reformation.
And how all of this is achieved? Through "exercises"—but "spiritual" exercises. St. Ignatius compares the Spiritual Exercises to bodily exercises like "walking, journeying on foot and running." If we think about it, we need to take bodily exercise in order to strengthen our bodily muscles, our bodily circulation, and the function of the heart. This is what the Spiritual Exercises operate in our souls: they are meant to strengthen the powers of our soul, to enlighten our mind, and inspire the will to operate more effectively. Everyone—without exception—is in need of conversion, and everyone also needs to make particular decisions in their life about how they are going to live—that is, what their vocation is, what sort of work they should do, etc. The Spiritual Exercises of St. Ignatius are a power means not only of refocusing our lives on God, but also of helping us to make God-centered choices in our lives.
St. Ignatius had a great knowledge of the human soul; he knew its nature, man's psychology, his weaknesses, and his fears. This is why the saint’s plan is applicable to any Christian soul, regardless of how advanced one may be in the spiritual life. In short, the Spiritual Exercises truly can help lead one’s soul to union with God.