The question and its answer succeed in getting Zuckerberg on record (in the Congressional Record, no less) as saying that his business model depends on advertising, with all that that implies about the relationship between Facebook and its users.

Think of it as like when you're on the witness stand and the attorney asks you a question. It's not because he doesn't know the answer. It's because he wants you to say the answer, to make it part of the public record and to have the jury hear it.

Speaking of juries, the arrogance of Zuckerberg's answer wouldn't win any points with a jury, either. He walked into a trap right there.

It's fun to bust on our parents & politicos but I'm pretty sure anybody who managed to raise you, or especially get into the Senate, has a much more extensive and nuanced understanding of life (and politics) than you think.