Idk, if you want to test people on how they understand formulae and order of operations without letting them just punch it into a calculator. The actual math isn't hard, but if you don't get substituting values into an equation then it's not trivial
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.
But if you want to buy only half a kg, you don't know how much it costs (if you dont know basic maths)- because it only lists the price for a full kg. Do you start pulling out your Excel for that?
Every offer lists how much per kg that offer is. So I do not need any calculations to see which offer is cheaper at all. Why would the price per kilo become useless of I only buy half a Kilo?