It’s not just about haveing a calculator, it’s also that it’s faster and more convenient if you can do simple sums like this in your head. It also means you can sanity check the numbers your calculator gives you to make sure you didn’t make a mistake entering the sum.
To your point below about products having their unit cost displayed, more than once I’ve seen that just be wrong, so I wouldn’t rely on it. Make sure you can check it in your head.