Joe S. Tudent's thinking:

Why should I be doing these optional problems? Pointless, no?

Predrag's answer:

EVALUATION METHOD: Overal course grade will be determined from the homework (40%), midterm (20%), and the final (40%).

By doing optional problem sets you can build up capital (ie, problem set scores over 100% right) which will help you through the rough times, problems you cannot or do not have time to solve. A wise investment.