Answering questions for us is easy. The further back along our timeline we go, however, the harder it becomes. What did we do before wikipedia? Well, you could go to a library and search for a relevant book on the subject. But what if your question is obscure or not simply phrased? You'll have to look for a little longer, perhaps doing some real research by skimming one book and using its footnotes to find another.
But what if the library isn't organized by the Dewey Decimal System? You will definitely need the help of a librarian and a master index that describes the organizational system of all the books. And what if the books don't contain footnotes? Now you're really getting into hazardous territory. You'll probably have to use the librarian as a living resource, relying on their knowledge of the contents of their library because they've read most of those books.
Stay with me. Now imagine that libraries are few and far between. It's not very easy to get to one, the way they organize their material is cryptic and only comprehensible by the people that work there, and there's no easy way to assemble a list of books based off of their subject matter. Your little questions more often go unanswered, unimportant bits of information being relegated to "whatever you can remember," or "what you heard the other day," because you can't really keep going to check some master database for what society claims is the most recent or most correct answer to that question.
Now, do away with public libraries altogether. Every library that still exists only exists through the will of either an individual donor or a powerful corporate agency such as a monastery or temple (or maybe a wizard's school if your wizards are into that). Reduce your literacy rates, so most people don't even care about the library.
How are you going to find anything out in a world like this? You certainly can't just think of a question and have it answered immediately. You need to consult someone who A) knows where the information you want is and B) knows how to get their hands on it. This is why sages exist in AD&D, and why the records that survive from the early and high middle ages were written almost exclusively by monks and priests. The books of the middle ages didn't even necessarily follow any organizational schema INSIDE either, simply whatever the author felt like writing at the time.
Information is not just a simple tool that you have access to whenever you want anymore; it is a highly valuable commodity that is damned difficult to get. Books are rare, hand-written, and regionally available. Even at a great scribal center, you're unlikely to find that book you want unless you have a copy with you already, because there's no guaranteeing that its ever been through those copyists. Many times, the fee of copying a book is simply to let the center keep a copy of their own so their library can expand.
Individual libraries have personalities based on their curator. One sage who is interested in dragons may have a huge section on them while keeping an anemic or non-existant section on great sea-beasts or history and wars. Each library is an expression of its curator. They cannot possibly be repositories of all knowledge with the means they have available, so instead they represent the things that individual people, real living librarians, felt were most interesting.
You can't think of any question you want and have it answered. Who was the king of this kingdom one hundred years ago? Well, that might be common knowledge but who was the king five hundred years ago? Huh, that one's a little harder. Better go and ask someone who knows what he's talking about.
Or else, better plan a trip to the library, which, in a medieval paradigm, can literally be an adventure in and of itself.