Why should we attend lectures ????????

In "A Beautiful Mind" movie when character of John Nash entered his class to give lecture he said he was wasting time by taking that lecture. I personally loved that dialog because I think why should I attend a lecture when I can learn those things from book.
If I face some difficulties then I can ask for guidance of the concerned person won't that save my precious time? So why all universities give so much importance to attendance? What can be the logic behind that when they have certain ways to test whether I am working correctly or not through assignment and exams.