George Herbert's poem called "Praise" contains a verse that has been taped to my computer monitor since I first read it years ago. Of all the creatures both in sea and land Only to man Thou has made known Thy ways, And put the pen alone into his hand, And made him secretary of Thy praise.
This is the answer to our question. Why is man on the earth? He is here to be the secretary of the praise of God.
To which our querying minds say, But do not "the heavens tell the glory of God" (Psalm 19:1), and not just man? Are not "the things that have been made" the secretary of his divine attributes (Romans 1:20)? Do not the "the trees of the forest sing for joy before the Lord" (Psalm 96:12), and "the rivers clap their hands" (Psalm 98:8)? Do not "the mountains and the hills break forth into singing" (Isaiah 55:12)?
Yes. They do.
[Read the rest of the article at Desiring God.]