To understand both Manoah’s question and the angel’s answer, you have to understand the customs about names in biblical culture. Knowing someone’s name was very important for several reasons.
First, a person’s name revealed something about his character. Thus Jacob was “heel snatcher.” Esau was “hairy” and Edom was “red.” Elijah was “My God is Yahweh.” Jesus was “savior.” Abraham was “father of a multitude,” and so forth. Not every name had significance, but most did.
Also, knowing a person’s name was believed to give people some amount of power over the person (we experience the same thing when someone shouts our name and we stop what we are doing and look, or if spoken to in first person by a stranger we get suspicious and ask, “Do you know me?”). It is also why Jesus has a name that no one knows but Jesus and God (Rev. 19:12). It was also why when a person had power over someone else, he changed the person’s name (Gen. 17:5, 15; 41:45; 2 Sam. 12:25; 2 Kings 23:34; Dan. 1:7). The angel would not tell Gideon his name, commenting only that it was wonderful, which most likely meant that it expressed qualities about him that Gideon had no business knowing.