In a basic mathematics for economists course one is exposed to the concept of directional derviative. Recall that a directional derivative is defined as:
$$\nabla f \frac{\mathbb{v}}{\|\mathbb{v}\|}$$
I know that it mathematically tells you the direction the function is increasing on a two or three dimensional graph, however I am yet to see an application of such a formula in any economics problems.
What are some applications of the directional derivative in Economics?