Removing green screen spill with expressions seems a little over the top. For the most part it is. Fusion has tools that are specifically designed for spill removal and spill removal.
If you want to learn the inner workings of expressions in Fusion though, this is a great exercise. It can get you more familiar with expressions, but also shows the kinds of things that are possible with expressions in Fusion.
VFX Artist Chetal Gazdar walks through the logic needed to create a green screen spill suppression system. That logic is then applied through Fusion’s node and expression system. Exercises like this can give you a greater understanding of expressions but also fundamental understanding of how tools like spill removal work.
It is impressive the level of control you can have in a node based compositor, expressions and some logic.