Some people may believe that going to classes should be optional, but I disagree. I don't understand how university students can expect to learn anything if they don't attend classes. Personal experience can help people learn about themselves and the world outside the classroom, but when it comes to learning about academic subjects, students need to be in class.
In class they receive the benefit of the teacher's knowledge. The best teachers do more than just go over the material in than class textbook. They draw their students into discussion of the material. They present opposing points of view. They schedule guest speakers to come, give the students additional information, or show documentary films on the subject.
Also, attending classes on any subject teaches more than just facts. It teaches students how to learn, how to absorb information and then apply what they've learned to other situations. Their teacher is the best one to help them with these skills. They can't learn them just by reading the textbook.
Going to class also teaches students how to work with the other members of the class. Many times students will be given group assignments. This is different from what they did in secondary school. Here they're with people from different backgrounds and experiences. In this situation, they learn how to handle working with people different from themselves to achieve a common goal.
Going to class also teaches students responsibility and discipline. Having to be at a particular place at a particular time prepares them for getting a job. Being at a place on time with an assignment completed prepares them for a career.
In short, by going to class students learn more than just information from the teacher. They also learn how to learn, how to work with others, and how to work responsibly. These are not optional skills in life, so attending classes should not be optional in college.