Why did God send John the Baptist?

The words of Gabriel to Zachariah echo the Word of the Lord recorded by Malachi, the prophet: "Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD. And he shall turn the heart of the fathers to the children, and the heart of the children to their fathers, lest I come and smite the earth with a curse" (Mal. 4:5-6).