Why does God have a day of judgment? Is such a thing necessary? What purpose does it serve? Why not just consign sinful unbelieving people to hell itself?
The answer is found in chapter 32. 2
The end of God’s appointing this day, is for the manifestation of the glory of his mercy, in the eternal salvation of the elect; and of his justice, in the eternal damnation of the reprobate, who are wicked and disobedient; for then shall the righteous go into everlasting life, and receive that fulness of joy and glory with everlasting rewards, in the presence of the Lord; but the wicked, who know not God, and obey not the gospel of Jesus Christ, shall be cast aside into everlasting torments, and punished with everlasting destruction, from the presence of the Lord, and from the glory of his power.
Two simple things:
1) For the glory of his mercy
2) For the glory of his justice
In the first, the elect are eternally saved – have everlasting life and receive fulness of joy with rewards, and in the second, the non-elect are eternally damned (condemned) – are cast aside into everlasting torments and punished eternally.