One reason that God sends suffering is to discipline His people when they are going astray. The Bible tells us that just as human fathers lovingly discipline their children, so God disciplines all who belong to Him. (Heb 12:5-11) Suffering will also help refine our character and build our trust in him.
It also exists because that is one way to bring people to Him. If your life is a smooth-sailing cruise, you won't seek God's help. These sufferings will provide an opportunity for us to show yourselves and others of His glory. A few bumps or two will help you draw closer to God and strengthen your relationship with Him as you find comfort in Him.
“Praise be to the God and Father of our Lord Jesus Christ, the Father of compassion and the God of all comfort, who comforts us in all our troubles, so that we can comfort those in any trouble with the comfort we ourselves have received from God. For just as the sufferings of Christ flow over into our lives, so also through Christ our comfort overflows.” (2 Cor 1:3-5). And thus we see that it also prepares us to help and comfort others who are suffering.
The early believers even counted it an honor to suffer for the sake of Christ (Acts 5:41, Phil 1:29, Matt 5:10-12). The apostle Paul wrote “we rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not put us to shame, because God's love has been poured into our hearts through the Holy Spirit who has been given to us” (Rom 5:3-5).
The Bible also shows us examples where God brought much good out of evil and suffering. In Genesis 37, Joseph’s jealous brothers betrayed him and sold him to some travelers who in turn sold him in Egypt to the Pharaoh’s captain of the guard. But rather than becoming bitter, Joseph faithfully served both God and his master and became his master’s most trusted servant.
Suffering and tragedy may also be sent by God as a warning to ungodly people. In such cases, though we may consider it to be judgment in one sense, it is also a mercy, an opportunity for them to realize where they are headed and repent before they are eternally condemned. Proverbs 29:1 warns us that “He who is often rebuked, and hardens his neck, will suddenly be destroyed, and that without remedy.” So when a person or a nation persists in rebellion against God, God will eventually bring irrevocable judgment.
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