Daniel 4:17

God also revealed the purpose of the judgment of this "tree." It was to teach all people that the Most High God (cf. Daniel 3:26) is sovereign over the affairs of humankind (Daniel 4:17; cf. Daniel 2:21; 1 Samuel 2:7-8; Job 5:11). He can, has, and will set up whom He will, even people of humble origin, to rule nations (e.g., Joseph, Israel’s judges, Saul, David, et al.). God does not need the mighty to do His work. Therefore it is foolish to become proud over one’s accomplishments and importance, as Nebuchadnezzar was.

God had sought to impress His sovereignty on Nebuchadnezzar previously (chs. 2, 3), but the king had not learned his lesson. So the Lord sent him a stronger lesson. This is often what He does (cf. Job 33:14-17). The last part of this verse is really a summary of the theme of the Book of Daniel.