How does Harriet's father's job assist the girls in their investigation in the book, The File on Fraulein Berg?

Harriet's father is a lawyer, and she is privy to information the other girls are not. She knows about secret dug-outs, for instance, the hideaways where the important people go in case of invasions, and she knows that German spies sometimes slip across the border from Eire. The girls learn that Fraulein Berg indeed did come from Dublin, and they know that she speaks perfect English.