For many students, the help they can get from their parents, teachers, counselors, etc. is invaluable. Parents should know the grades their kids get so that they know where and when to help. Knowing more than just a quarterly letter grade facilitates pinpointing issues before they worsen.

There is much more homework in high school than there is in middle school. High school homework is also much more challenging. Someone who developed good homework and study habits early on would still need to work on those habits throughout high school.

And as Gordon said in this blog post: If you get the best grades you can, you will be proud to let your parents know.