The doctrine of predestination is a difficult topic. I would be the first to admit that I have not mastered it.
While I may never fully understand every aspect of the doctrine, I have found two purposes (or goals) for predestination.
1. God has predestined Christians to be conformed to the image of his Son. And we know that for those who love God all things work together for good, for those who are called according to his purpose. For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. And those whom he predestined he also called, and those whom he called he also justified, and those he justified he also glorified" (Romans 8:28-30). The Lord is at work in the life of a believer, shaping circumstances and providing guidance through the Holy Spirit, so that the disciple of Christ can think, feel, and act more like Jesus Christ every day. This is a life-long process of progress and occasional setbacks, but God is determined to see every one of his children develop into the image of Christ.
2. In conforming followers of Christ to the image of his Son, God has predestined us to bring him glory through our continually transformed lives. "Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us for adoption as sons through Jesus Christ, according to the purpose of his will, to the praise of his glorious grace, with which he has blessed us in the Beloved" (Ephesians 1:3-6). As we grow in holiness, humility, grace, mercy, justice, kindness, and every other attribute of Jesus, we lead lives that reflect well on our Savior and bring him the glory he deserves.
Our destiny as Christians is conformity to the image of God's Son so that he may be praised for his grace toward us. It's why we were predestined for adoption into his family.