Why is it impossible to teach spells verbally (or is it?), why do you have to read a tome and why is it destroyed?

Before Skyrim, you buy spells directly from a mage and not through tomes. I assume this implies that you get taught the spell verbally, but I'm not sure.

I always saw the tomes that you read to be taught as a gameplay mechanic and imagine that in reality you would be paying for a training session to be taught the skill by the mage.