It’s MUCH faster to operate the calculator from the keyboard than by clicking on it with the mouse. If you have a keyboard with a numeric keypad it’s super-quick to enter numbers, but you can use the number keys in the top row of your keyboard too.
Plus and Minus are easy to find on the keyboard, but Multiply and Divide are a little harder. Programmer-types will know that the asterisk (*) is used to multiply, and the slash (/) is used to divide. Now you know too.