In fact, instructors are not a requirement for learning. We learn from our environment, and from observing others.
Teachers are role models, demonstrating by example how to learn more effectively.
I would also say that teachers are mediators between students and a culture's storehouse of knowledge. In an oral culture, that storehouse is memory, so the best teachers are those with the most memory, the elderly. Take a close look at what's going on in universities across the country today, and you'll find that the business model has become dominant, the main concern being cost effectiveness of faculty. There's nothing that administrators want more than to get more for less, and they see distance learning and related technologies as one way to get it. They'd eliminate teachers altogether if they could get away with it.