I know a hiring manager who asks his (engineering) candidates what is 20% of 20,000? It's amazing how many engineers are completely unable to do this without a calculator. He said they often cry. Of course, they're all "no hire".
How did they get a degree, one wonders?
To quickly calc 10% just multiply the number by 0.1 which you can do by moving the decimal point one place 20,000.00 => 2,000.000 then it is easy to double that number.
to get 4,000.
17% for example is 1.7 x 10%
in this case 1.7 x 2,000 = 3,400
You mostly have common equivalences like this in your memory and you can be faster than computing the actual thing with arithmetic. Or have good approximations.
And yes, it's incredibly useful in enabling recognizing when your calculator gives a bogus result because you made a keyboarding error. When you've got zero feel for numbers, you're going to make bad engineering decisions. You'll also get screwed by car dealers every time, and contractors. You won't know how far you can go with the gas in your tank.
It goes on and on.
Calculators are great for getting an exact final answer. But you'd better already know approximately what the answer should be.
Humans are much better at pattern matching than computation, so the safest solution is probably to just double check if you've typed in the right numbers.
It might be counterintuitive, but the cheaper (and therefore successful) solution will always be more technological integration, not less.
In this case, better speech recognition, so the user doesn't have to type the numbers anymore, and an LLM middleman that's aware of the real-world context of the question, so the user can be asked if he's sure about the number before it gets passed to the calculator.