A lot of homework is just busywork. It also cuts into the time a child should be spending with their family or just being a kid.
Also, a lot of homework in college, especially in the sciences, come in the form of optional reading and practice problems. I was so used to doing worksheets that would get graded all my life that, when I got to college, it was difficult learning how to study for a lot of my classes. I would end up spending so much time doing all the practice problems even on material I was pretty familiar with. What assigned busywork does is that it does not allow a child to discover what works best for him or her.