Paul must repeat this truth because it is of the essence of man's sin that man boasts. Sinful man's assertion of autonomy, his declaration of independence from God is a proud, rebellious quest for apotheosis. When God saves us from sin, He must also save us from the proud heart that is the source of sin. All of our works are therefore excluded either as the ground or the means of our salvation. In all of this man is passive, a recipient.
Salvation is a gift.
But good works are important. We are God's masterpiece, remade in Christ so that we will be able to do good works. True good works, rather than a ground of boasting or self-exaltation, are a means of praise to the God of all grace who has blessed us with so great a salvation. They express gratitude. To be lacking in good works is to lack the living power of God in our lives.