Supposing him to be a man and a prophet, to whom he would in this manner express his respect, as was usual to strangers. See Genesis 18:5 Judges 6:18.
Related Commentaries of Judges 13:15
Judges 13:15
15 And Manoah said unto the angel of the LORD, I pray thee, let us detain thee, until we shall have made ready a kid for thee.c