How does it show if someone prepared for this interview, rather than just having prepared for generic interviews?

Being prepared for a particular interview (i.e. knowing about the company you're interviewing with and having relevant questions) is probably a good sign that the person wants that particular job, which is generally a good thing. But simply being prepared for 100 Common Interview Questions doesn't tell you that — and might even be more a sign of the opposite.

It's useful for filtering companies. Last time I was looking for a job I had decided that stupid questions like this would be the end of my considering that company. Thankfully the place I'm at didn't ask stupid questions.