[totalc]

Graded homeworks might be used as checkpoints verifying that students are learning key concepts, prior to the exams that are a larger component of the grade. Without these graded checkpoints, lazy students given only voluntary homeworks might reach the exam with little preparation

(lazy student speaking from personal experience. being graded on the homework 'encouraged' learning the concepts!)

[dagw]

lazy students given only voluntary homeworks might reach the exam with little preparation

And that is bad, why exactly? You're unprepared, you fail, you learn a lesson, you retake. Repeat until done. I too was a lazy student and found that failing a few classes and having to resit the exam was a fine way to motivate you to study.