A calculator gives the answer. Understanding tells you when the answer is obviously wrong.

A machine will happily hand you a number that is off by a factor of a thousand, because it does not know what the number is for. That check lives in you. Understanding is what makes you look at an answer and think, a room cannot be four hundred feet long, so something went in wrong. Without it you are trusting your own typing, which is the part most likely to fail.

Before you read the screen, say roughly what you expect: about twenty, somewhere near a hundred. Then compare. One second of guessing catches most of the mistakes that cost points.