"what if your computer breaks?" is a poor reason. The real reason is to get an estimate of results in your head so you can cross-check the computer for mistakes

"what if your computer breaks?" is a poor reason. The real reason is to get an estimate of results in your head so you can cross-check the computer for mistakes. How much ice cream to feed my city of 30,000? If the computer says 7.336E9 kg, that's a problem.