If the time spent while at school was efficiently used to teach and study then there would be no need for homework. I think many teachers are shirking their duties by assigning homework. Keep the students hard at work while at school and then leave them alone when the final bell rings.
As best I can figure it, homework is now assigned so the parents can teach the kid. That way, the teacher won’t need to do so.
FWIW, I did very little homework in high school. However, I took calculus and used CLEP to get rid of my general ed requirement in college. I’m puzzled why it is that I could learn without a ton of homework, but my kids (and now grandkids) need hours of it every day.