Because the person asking the question has already shown that there is at least one thing he/she doesn't know, giving the other person the chance of playing omniscient?
Or perhaps because the person answering is looking beneath the surface of the question asked in order to answer what is perceived to be the deeper question?
Or perhaps because people generally tend to think they're smarter than any other random person?
If I ask you for your opinion on steak, I don't really give a damn about your opinions on fish. I asked you about steak!
But what if I really want to talk about fish? Isn't it then obvious that your question about steaks is a really good way of getting into the far more interesting topic - fish!
If I ask you a specific question about HP-brand media, I don't give a damn about your opinions of other media. I asked you about HP media!
Isn't it nice to have an opportunity to let off some steam now and then!