I don't think there was ever a solid use case for graphing calculators in school, at least not in my experience? The curriculum didn't make good use of them and I'm not convinced it could have. There's little value in having every kid in the classroom replicate the same plot of y = sin(x) or whatever on a tiny screen. And other than such demonstrations... what are you gonna do with it? It was never flexible or powerful enough for serious math. You weren't going to run circuit or physics simulations on a TI-89.

There are other features that can be useful - scientific notation, symbolic solver, unit conversions, etc - but graphing as such always seemed like a gimmick.

I think it's more of a not-entirely-rational appeal to parents: "if my kid has a top-notch calculator for high school / college, maybe they're gonna be better at math". And kids did not object, but in the end, mostly just sideloaded games and horsed around.