Price / quantity calculator lets you compare products with various quantities and prices. So, if you're ever in a shop trying to decide which product is the best buy (is sold with the lowest margin), this is a tool for you.
For example, you want to buy some flour. You can choose between a smaller package - 1 kg for 1$ and a bigger one - 1.5 kg for 1.35$. Which one is a better buy? Thanks to using price / quantity calculator you know that the unit price in a larger package is 0.9$. The "You Save" box tells you how much more you'd have to pay if you bought the same quantity at the price per unit of a smaller item - in our example - 0.15$. If you want to know how much you will save on an offered discount you might want to check out the discount calculator.
In some countries, the price per quantity information is provided on the price sticker so you might want to take a closer look and not need the calculator at all. Still, you might find yourself in a situation where that's not the case, for example when the pricing comes up in a conversation.