To be honest, I don't think there should be a rule about home work. Let's forget about home work, let's call it practice.
Children learn a new theory, they need practice to really understand it. Howerever, children are different. Some children maybe only need to practice once to get it, some children maybe have to practice a few times. Ideally, children can learn and practice it all in the school time. However, there is only limit time during school hour,some children may not have enough practice time. Also, as there is only 1 teacher and 1 teacher assistant, for 30 children, sometimes it is hard for the teacher to know every one to see if they fully grasp the stuff taught.
So, by doing some homework at home, children get practice, also, parents can check the answer or check teacher's marking to know the weak area, so can help if needed.
For primary school, the stuff to be learn is simple, that's why there is no need for homework for some children. But you have to accept, if your children is below average, you have to get them practice more at home to catch up if you care about their education.
For some able parents, no matter there is homework or not, they can always help, they find stuff to do themselves. But for some parents, they don't know about education system much, the homework set up at school would help them to help their children.