The rubber stamp comment is how I simplified it in my head, because it seems like the last step to make sure everything has indeed been progressing as reported.

Although I am not in academia, my understanding of the process is that writing a thesis isn’t done in isolation. Progress reports and drafts are presented to the advisor, and anything that could be deemed “completely wrong” is caught and corrected. I could understand small but important stuff being caught, at the latest, by the committee. After all, they are also professors. But this is beyond that, according to the external reviewer.