ExDemMom said it above;
It's the lack of proper and complete (which is honest) American history being taught.
When my first wife and I started homeschooling twenty something years ago, we were both a little surprised at the continuing history of Pennsylvania that went from year to year in the curriculum we subscribed to.
It was way back then that I began developing my opinions about American history and the lack of it being taught in public schools that had in part, motivated us to want to home school.
I was educated in the fifties and early sixties.
I joined the Army in 1965.
When I got out there was a full scale social riot going on that was never really corrected, just quelled.
I think I could make an argument that part of the angst felt by returning vets is because we were 1950's, post WW2 victory educated and had been forced (or enlisted for idealistic reasons) to fight a war that was no where NEAR what our history had taught us about war, America, freedom, etc.
VVAW (Viet Nam Vets against the War) was born from that sentiment (with a LOT of communist (at the time) assistance in colleges)
History is the key to understanding, in my opinion.
None of which answers the original question:
What meaning do mandatory pledges have?
I say none. Forcing kids to say the Pledge is just stupid, they’re mouthing words they don’t understand and don’t believe. It’s frankly a stupid idea that goes against everything America stands for. America is about freedom of choice and the freedom to NOT blindly pledge allegiance to a government that’s supposed to work for US. By the people, for the people and of the people. Government officials should be pledging their allegiance to us, not the other way around.