Karazhan Crypts: Why was it made?

I've heard of the Karazhan Crypts, but I only started playing a little over a year ago, so I don't know a thing about it. For some reason, the majority of the forum responses are pretty vague. Could anyone shed some light about that place? I've seen videos of it, so I don't need to hear about what's IN there, but I'd more like to hear why it was made and why it is blocked off.