Why are believers supposed to do good works?

Believers are to do good works because they are God's workmanship, created in Christ Jesus for good works, which God beforehand ordained that they should walk in them. A believer is to live differently from the lost, practicing these good works as the purpose God prepared.

Les teaches that we are His workmanship and that God has formed us. He uses the illustration of a symphony—instruments coming together in beautiful harmony, with major and minor chords—to show that it takes effort to put together, and that is what God has done with us. Because believers are created in Christ Jesus for good works, and God before ordained that we should walk in them, Les says a believer is to live a different life than the lost person and practice these good works.

Ephesians 2:10

For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.

Why did God create believers in Christ Jesus unto good works?

God ordained beforehand that believers, having been created in Christ Jesus, should be the practitioners of good works and walk in them. This is the way He set it up and planned it for the Christian community, so that good works flow from the Christian life built on Jesus Christ, the foundation.