an underappreciated game design skill is the ability to just... sit and think through all the implications of a system by simulating the entire game in your head. you can't just think through the consequences of a change; you need to think through the consequences of those consequences
Any time you're designing something, you should be thinking of the implications of your design choices, and what they might mean down the line. How they interact with what came before, and what will come after.
What if your client suddenly wants you to turn all florbs into purple blops? Can your design handle that? Will Future You — or whoever ends up being the poor sap that needs to implement the thing — curse Past You, or will it be a breeze?
And if a user feeds the purple blop input a green smarg, what'll happen then? Because you know someone will. Will your design fail gracefully and be informative, or will it just blow up and leave a small crater behind?
These are all incredibly important things to consider with every step of the process.
So yes, extremely valuable skill to have, and unfortunately, quite a lot of people in our chosen profession just... don't have it, and don't seem to want it either.