[Froedlich]

Back in the (early 1960s?) Isaac Asimov wrote a book on how to do arithmetic calculations in your head. He pointed out that someone who developed that skill could solve problems faster in their head than by reaching for a slide rule, and they wouldn't necessarily be limited to slide rule approximations. Engineers, accountants, etc. would be more productive by being able to do calculations without having to reach for a slide rule or pencil and paper.

[pause for 'slide rule' chuckle]

The same applies to "open calculator app and key the figures in," though. I'm not sure that "user interface" was a thing back when he wrote that, but that's what he was talking about.

[tkgally]

That’s one reason the abacus hasn’t been completely replaced by calculators: some abacus users become able to calculate very quickly in their heads by visualizing the changing bead positions. In Japanese the skill is called 暗算 anzan, literally “dark calculation.”