Imagine I have several drop shadows, with different colors, but same offset and blur values. Now I want to change the horizontal offset for all of them at once. Even if I use styles, it would require me change each style I have manually.
With Appearance variables though, I could have use something like '%h_shadow' in effects’ values’ fields, in all of these shadows, and set it equal to an actual numerical value in one place.
So, once I change it, every other shadow will adapt this immediately.
CSS has variables, saves a ton of time. Imagine modern web without these!