Those verses throw a light upon the parallel passage in the book of the Kings. While Solomon addressed the people he stood before the altar. But when he came to address the Lord he fell upon his knees.
Related Commentaries of 2 Chronicles 6:12
2 Chronicles 6:12-13
12 And he stood before the altar of the LORD in the presence of all the congregation of Israel, and spread forth his hands:
13 For Solomon had made a brasen scaffold, of five cubits long,a and five cubits broad, and three cubits high, and had set it in the midst of the court: and upon it he stood, and kneeled down upon his knees before all the congregation of Israel, and spread forth his hands toward heaven,