Haggai's unique title as the messenger of the LORD (the same Hebrew phrase often translates to angel of the Lord) gives him special authority as the Lord's representative.
I am with you: This confirms God's promise to the people of Judea, showing his presence and support in their building project (Haggai 2:4–5; see Genesis 26:3; Exodus 3:12; Isaiah 41:10; 43:5; Jeremiah 30:11).