Why does Herbert suggest that his father wish for two hundred pounds?

Herbert suggests that his father wish for two hundred pounds to illustrate the idea of being careful what one wishes for. He believes that a specific amount would allow for a more manageable and realistic desire, rather than leaving it open-ended, which could lead to unforeseen consequences. This reflects the theme of the story, where the impact of wishes can be both transformative and dangerous. Ultimately, Herbert's suggestion highlights the blend of hope and caution in the pursuit of one's desires.