What is the benefit of using formula functions on a spreadsheet rather than just simply entering the answer?

Excel is far more powerful than a calculator. There are a huge amount of things it can do that a calculator cannot do, far too many to mention. At a very simple level, for doing basic calculations Excel is far better. With a calculator, you can only see one number at a time. If you have a lot of figures to add, you won't be able to see them all, like you can with a spreadsheet. So it is far easier to see what you are doing and to spot any errors. You can use the same figures to do lots of calculations, and not have to type them in over and over again. Excel provides a wide range of built-in functions to use, that a calculator can't do. So there is no real comparison. Excel is a far better calculator than the Calculator on your computer is. Unless you are doing an extremely basic calculation, like adding two numbers, it is much better to use Excel.