According to my friends from Heritage, 50% of your internal marks depend on your attendance. So, your attendance plays a significant role in your internal marks. Viva is conducted by the teachers as a part of the internal. Scoring good marks only in the end-semester examination will not help you in scoring a high GPA, you will have to score good marks in internal exams as well. If you have a good attendance record, you might get grace marks as well.
Since it is an autonomous institute, you will have to attend the classes. It does not matter whether you like the classes or not otherwise you will not get good internal marks. This is why there is a 60% mandatory attendance criterion.