When to use gradient and when to use only one coordinate

Hi,
I was wandering, sometimes in physics, to get acceleration from a velocity time graph, you would have to find the gradient of the tangent of the curve. But in other graphs like say Voltage current graph, if you want to find the resistance at any point (Which is V/I) you simply take the coordinate of that point and just divide Voltage by Current. Why is it that we took there the gradient of a tangent and here just the coordinates although in both the Gradient represented acceleration and Resistance.

Thanks :))