I would guess that you've never been a teacher. The problem is that any teacher has a classroom with students with abilities and aptitudes that range between some Low and some High. The teacher must always teach to the middle kid in that spectrum. Going slower/lower bores the kids in the top half and going faster/higher completely loses the kids in the bottom half. Half of the kids may just "get it" and not need help but the bottom half will need help that the school system can't provide. Note: I don't condone the parent just doing the homework for their kid.
The problem here is that the parents best able to help their children are more likely to have kids in the top half (who don't need help) and the ones least able to help have the kids who need help the most. This can be solved with a restructuring of our school system but very few schools are willing to try. Instead we stick with the ridiculous age == learning level method.
I am not a teacher but I did volunteer in my children's classroom when they were young. I do understand what you mean by the range in comprehension.
I am not counting the other things a parent does to enhance a child's learning experience as homework time - so we all might be saying the same thing.
I will give a small example which might illustrate having an enhanced learning experience. One of my volunteer assignments was to teach children math using coins. My son and daughter were naturally motivated to understand how much money they had because I had them save money buy things at a very young age. I was shocked that some classmates did not understand denominations and others could not do the math. My husband made coin dice for the classroom kids. They rolled the dice and added the values to figure out how much money they had on the coins. This was a big hit with the kids.
What I didn't want to do was do homework for my kids or give answers. Discovery is how children learn. My kids ended up proud of what they could do on their own. When they failed - we discussed taking ownership of failure.
The problem here is that the parents best able to help their children are more likely to have kids in the top half (who don't need help) and the ones least able to help have the kids who need help the most. This can be solved with a restructuring of our school system but very few schools are willing to try. Instead we stick with the ridiculous age == learning level method.