Our HS also asked parents to provide a letter sharing info that the GC might not know about your child. For instance, I shared that one son earned Eagle and another was a gallon blood donor.
In terms of colleges, when my D was applying for schools, SUNY Geneseo asked for a parent letter. I did one and my friend didn’t. My D didn’t get in and hers did, but her D also had a much better GPA and more course rigor, so even if our letters were equal, it wasn’t enough.