In this drag and drop interaction, when you pick and item from the shelf and drop it in the shopping cart, the price of the item is added to the total amount on the cash counter display.

What is that example intending to do? When should the user use such a theme?

In this example, you will see the use of drag and drop interaction with the ability to do mathematical operations. In this example, only addition has been used. For your projects, you can also try subtraction, multiplication, and division, using the same method.