[oytis]

> you just need to understand when a calculator is not giving you what you expected

How do you do that if you can't do arithmetic by hand though? At most, when working with integers, you can count digits to check if the order of magnitude is correct.

[lordnacho]

You can do arithmetic by hand without being fast or accurate. It's still useful to check that calculations are correct, it's just slow for the ancient use of tallying up a bill.