>Until high school, there's literally no reason for the kid to be practicing math after school. //

Maths is fun. That's a reason.

Maths is useful and some children can't pay attention in class. That's a reason too.

Analytical and creative thinking expands the mind, ergo maths.

Different kids have different needs. Busywork for homework certainly isn't useful except as a way solely to keep a child occupied.