Answer:
If you type formulas with numbers directly into them, then every time you make changes to the spreadsheet, you will need to directly change the formulas too. It also makes them more difficult to copy. It also means that you will not see the figures that are in the formulas, just the result. If a formula references a cell, then you can see the figures that are used and the cells can be changed without having to adjust the formulas and the formulas can be copied to other cells and will look at the correct cells to do other calculations. For those and other reasons, you use cell references and not numbers in formulas where possible.