Economists have modeled education as serving no purpose besides providing graduates a credible signal of their intellectual ability. Some employers have trouble assessing the quality of potential workers, but at least they can deduce something about the candidates based on their ability to get into and graduate from top schools, even if academic success is not perfectly correlated with workplace success. It doesn't matter what the students actually learned, as long as they had the follow-through to graduate. The most famous paper on this topic is Michael Spence's "Job Market Signaling," published in 1973.
Another function of education is accountability. As one professor pointed out to our class last year, we could have all found the syllabus online and read all of the course's books ourselves, so what's the point of doing it in a classroom setting? By enrolling in school (especially if you're paying your own way), you're setting up a situation with bad outcomes if you don't succeed. Whenever you hit a rough patch in the material, these consequences motive you to get through it, whereas otherwise you might have given up, if you were just learning on your own. And if we concede that being around a teacher and fellow students really does help you learn, this increasing your chances of success ever further.
I recently started taking a course in linear algebra (not at George Mason). The professor does nothing more than walk us through the textbook, page by page and sometimes word for word. He has other annoying habits, such as writing whole paragraphs on the white board or telling us about how he keeps getting fired. My classmates complain about his teaching methods, and he's definitely one of the worst teachers I've ever had. But if nothing else, I'm going through the textbook to answer his homework questions (or rather, the textbook author's homework questions), something I would have had a hard time motivating myself to do if I had just bought the textbook and put it on my coffee table, promising myself that I would get around to studying it someday.