I stumbled upon the notion of dynamic resistance of the p-n junctions.

I'm pretty good at calculus so I understand what dynamic resistance is. What I don't understand though is why we take it as the resistance of, say, a diode for small ac voltages instead of the static resistance.

Isn't the current through the diode the voltage over its static resistance? Why do we use the dynamic resistance which is essentially the rate of change of the current in regards to the voltage? Why do we care about the rate of change and don't care about the value of the resistance itself?