If works are not necessary, there's nothing the church can hang over the members heads to keep them in line. If it's by works, it's really a control issue. The organization then has something with which to control its adherents. You have to do THEIR good works, partake in THEIR sacraments, give to THEIR church, do what THEIR leaders say.
If it's by grace, the person is free. Free to do as the good works that God prepared for them to do for the honor and glory of God, free to think for themselves, free to worship somewhere else is they so choose, and free to do what God calls them to do.
1 Corinthians 11:3 But I want you to understand that the head of every man is Christ, the head of a wife is her husband, and the head of Christ is God.
Romans 14:12 So then each of us will give an account of himself to God.