First, homework lets the student apply the knowledge in classroom to the problems. They will try it again and again in different setup. It's much easier and cheaper to learn in this way, than to learn from their loss in games.
Second, class is only once a week. On the other 6 days, homework will remind them what they learned in classroom.
Third, a student who does homework is often better than a student who doesn't. I can easily tell who didn't do when we go over homework. If a student finish her/his, s/he usually wants to speak up in class, otherwise s/he just looks down at her/his workbook, pretending that s/he is checking or thinking. Good participation will help her/him learn more, and motivate her/him do better next time.