If you read the company's web site, and are a curious and interested person, then you should pretty quickly stumble upon things you would like to know which aren't written there. So if you come up empty, either you are not interested in that company at all, or your curiosity is so easily satisfied that a plain web site can answer all your questions. Both is not a great sign for a prospective employee.

Even suppose you apply to a straightforward company with an extremely detailed web site, so that all your questions have been answered already, then this is still a test if you can meet social expectations. You are indeed expected to have questions, and if you don't even follow this simple convention because you think you know everything already, then your social skills are probably underdeveloped.

Finally, even you already know much about the company (e.g., about the most important product), asking about things you know already and comparing this with what you are told in the application talk will give you additional valuable information. Are they excited about their product? Are they exaggerating? Are they bored when answering? Do they know the basic information on their own web site? If you don't use the opportunity to extract as much information as you can, it's simply not smart.