Why would a student go to a lecture at all if everything was in the book already? Why would a student go to a lecture at all if everything was in the
  book already? The lectures make sense if and only if the lecturer
  tells more than in the book(s) and in more detail (or at least, in a
  better/more clear way). [not my comment]


Especially in light of the Internet in general, what is the added value of a lecture over a (hypertext)book?

Reasons I can think of:


spoken word might stick better, because our language is not primarily based on text,
the possibility to ask for clarification,
some material might not translate well to media


I dislike those arguments, because a huge class can neither run at a comfortable speed for all at once nor answer everyone's questions. The expectation seems to be that the students should be homogeneous and everyone nonconforming should be filtered out.


personal contact affects us on a deeper level, role models are important


Depending on the class size, even that is doubtful.


to form communities,


which would substitute or supplement the professors, but obviously at a different rate.

This is a trick question, because it depends on the style of the lecture.  In my experience the lecturer is acting as a medium and I'm watching them half the time writing on the board, or someone is asking obvious or distracting questions.
I think something mentioned in a comment is far more important than usually acknowledged: the lecture, standing at a definite time and place, plays the role of a ritual: at that time and place, the students (should) only focus on learning. This gives a rhythm, a time frame to help pace one's study.

In my experience, having very lenient rules such as long delays to turn in homework leads most student to postpone until the very end, and thus to work less than expected (to be honest scholars often do the same with e.g. grant applications). Similarly, leaving students to work with books would probably not end up well unless there are periodic incentives to have finished some chapter by some time. In fact, it seems even MOOCs tend to have a schedule, with material released periodically and only available for a given period, to force students to work it in a given time frame.

Another point is that a lecturer in fact makes small adjustments to the lecture in response to low-level signals from the students. Very concretely, it happens even in large amphitheaters that I feel that students all got a bit lost, which leads me to repeat a point or slow down. 
A comparison made by Krantz in his book on mathematical lecturing struck me: when one drives a car on a mostly straight road, one still steers the wheel, even if very softly. This almost imperceptible steering makes the difference between getting to the end of the road and getting off-road pretty quickly. Letting student only study on books would be like launching a car without a driver (nor a computer driver).

Last, any given course has a precise context (background of students, future study, other parallel courses, goal of the lecture, etc.) which is always even slightly at odds with any given book. The lecturer adapts the lecture to her or his precise goals and to the context. In particular, indications of the most important points to remind of every lecture can be very useful.

All that said, lectures in front of a large number of students are not a very adequate way of teaching - it is often the best we decide to afford.