We used them in 10th grade "advanced" math (Canada, I think the equivalent in the US would be AP?). TI-80plus in the early 00's. We didn't have laptops in classes.
Main use case was quadratic equations, which were a significant part of the class. We could plot them to get an intuition of the effect of the parameters. Plot them to visualize the zeros (or the absence of zeros), or the minima/maxima to get intuition on those. Same for finding intersection points of a quadratic function and a line, or two quadratics. We also programmed the quadratic formula in it, to get faster results instead of typing it out each time.
Other parts of the class were trig and stats, from what I remember I don't think we used the graphing/programming much in these parts.
Main use case was quadratic equations, which were a significant part of the class. We could plot them to get an intuition of the effect of the parameters. Plot them to visualize the zeros (or the absence of zeros), or the minima/maxima to get intuition on those. Same for finding intersection points of a quadratic function and a line, or two quadratics. We also programmed the quadratic formula in it, to get faster results instead of typing it out each time.
Other parts of the class were trig and stats, from what I remember I don't think we used the graphing/programming much in these parts.