But, consider you're stranded in the wild. All technology lost due to an accident. It's just you, nature and your skills. How will you know then for how many days the melons you've foraged will suffice if you've found N of them and eat one a day? /j

That was a short answer that turned out to be mostly wrong. The longer answer is "if you don't understand how this works, you won't have the intuition to notice when you get absurd results from the calculator". If you don't have that intuition, then when you inevitably make a small usage mistake on a calculator (or in matlab or wolfram alpha or whatever), you'll end up not realizing that you got a clearly wrong answer.