What advantage is there to using algebraic equations instead of numerical values when defining the dimensions of a CAD models?

Let's say you wanted to design a series of boxes that could nest inside each other. Instead of specifying the height, width and length of each box, you could define them by saying the length was 3x the height, and the width was 2x the height. Then, you would only need to specify the heights of the boxes and the software could figure out the rest.

This is a really simplified example, but the principle can be applied to more complex designs. For example, a door manufacturer offers a specific model of door in multiple heights and widths. But the plans for building a door can have all elements defined by specifying a height and a width, with the rest of the design (lengths of stiles, rails, panel sizes, etc.) calculated from those basic dimensions.