Judges 13:21
But the angel of the LORD did no more appear to Manoah and to his wife. Then Manoah knew that he was an angel of the LORD. But the angel of the LORD did no more appear to Manoah and to his wife. Then Manoah knew that he was an angel of the LORD.
After the angel ceased to appear to Manoah and to his wife, Manoah understood that the visitor had been the angel of the Lord. That recognition filled him with awe and fear at having been brought near one sent from God's holy presence, showing reverence before God's majesty and power. He concluded that having seen a messenger from God's holy presence meant that he and his wife would die.
God employs angels to communicate with human beings and to assist His chosen messengers.