Lets take a moment to think about the purpose of homework, using maths as our main example. Homework is designed to maintain, rather than extend, student learning. Students learn a skill at school, like how to do long division, and then take home work that allows them to practise that skill. Once they've practiced that skill in various circumstances (i.e. through applying it to a variety of different styles of questions), they are ready to move on to a separate skill.
If students don't do their homework regularly, they lose the skill. This means that when the class moves on to something else, these children are left behind. And if they continue to neglect their homework (or if they are not given homework at all), they can fall further and further behind. Furthermore, if the skill is a fundamental skill like times tables or division, a students' inability to easily use this skill will have a detrimental impact on everything else they do in this subject. Their speed and accuracy will be affected, and as a result they are likely to be labelled incapable or unintelligent, when in reality all that's happened is they've missed out on developing a solid foundation in the core mathematical skills.
Think of it this way: when your child learns a new skill, they are developing new pathways in their brain. When they practise that skill over and over again, they are wearing that trail into a well-paved road that is easy to find when they need to use it. Homework is there to pave the road and widen the lanes so that when we go back to use it, we can find our way to the solution with as little fuss as possible. Using this same analogy, we could say that a desired outcome of homework is that it allows us to follow our well-paved mental roads to new skills and problems with limited roadblocks.
However, not all schools are giving adequate homework and not all schools are giving relevant homework. That is why parents need to be aware of their children's homework schedule and be willing to create or provide supplementary homework if need be. It can be hard to initiate a new homework schedule or program at first, but keep in mind the desired outcome. A little extra effort now can mean a world of difference to their future learning experiences, as well as improving the opportunities available to them.