I didn't read all the comments, so this might have been said already. But I think there are some professions where quick mental arithmetic provides an edge. At

I didn't read all the comments, so this might have been said already. But I think there are some professions where quick mental arithmetic provides an edge. At one point I was learning how to play professional poker and while I had the common situations memorized, I'd calculate the less common situations. There are more use cases than just poker, chance estimation in general, or in a casual discussion where the other might not have a lot of patience. Compared to the old days however it has been in decline, but just like writing -- even cursive writing -- it still has its uses (cursive writing: exposing yourself to multiple ways of writing allows for a more fluent writing style in my experience).