The reason is, “that we should bring forth fruit unto God.” Not only does the fruit tell what kind of tree you are, but the fruit of a “good” tree is for the Glory of God; “fruit unto (or with reference to) God”; which is a fruit that glorifies Him and not the flesh (Galatians 5:19-23).
Jesus told the Apostles why He made the tree good. Of course it is the same reason He made the apple tree; yes, to have apples, to have fruit of its own kind. John 15:16 “… that ye should go and bring forth fruit, and that your fruit should remain…”
The good tree’s purposes are to:
1) “Go”; that is to go or to walk according to the instructions of the Master, the Lord Jesus Christ. The “good tree” is to walk by Faith (or by Christ, Who is The Faith; Galatians 2:20); for “…The just shall live by faith” (Romans 1:17); now, in this life, through His life; and in the future, through His life for eternity (Romans 5:10, 17). Christ is Eternal Life (1 John 5:20; John 17:2-3). So we are to “go” in the Power of His Life, as His witnesses in a wicked world (Philippians 2:15-16).
2) “Bring forth fruit”; this fruit is the Work of the Holy Spirit. (Galatians 5:22-23) “But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, (23) Meekness, temperance: against such there is no law.” The love of God, which is shed abroad in the heart by the Spirit of God in Regeneration (Romans 5:5), is the Core of the Fruit of the Spirit of God. This is where all other aspects of His Fruit begin. The bearing of fruit is not optional for the servant (and friend) of Righteousness; for Christ said in John 15:16, “Ye have not chosen me, but I have chosen you, and ordained (appointed) you, that ye should go and bring forth fruit...” Also in Romans 6:22, “But now being made free from sin, and become servants to God, ye have your fruit unto holiness, and the end everlasting life.” Paul does not present “fruit unto holiness” as optional, but as a natural progress; as the apple trees bear apples, so the servants of God bear fruit “unto holiness, and the end everlasting life.”
Do you live to bear the Fruit of Righteousness? Some people live to eat, while others eat to live. The main purpose of living for a child of God should be to live to bear fruit unto God; to be a light set on a hill; to be a witness of the Purpose of God for His Body, the Church; and not to live to reap the benefits of God. But to live to be a witness of the Grace, Mercy, and Love of God to sinners; to declare that God can, and does, and has, by His Grace (Ephesians 2:8-9), make the corrupt tree an incorrupt tree; that is, a “good tree” that bears good fruit unto God.
What is the reason you live? Do you live to receive; or do you live to give? God has called His sheep to go and bring forth Fruit of Righteousness.