To extend this example, I could place the same numbers in A3 and B3. This time, however, instead of using the cell references to add together, I could manually, just add the numbers together in C3 as displayed below.
While this will get me the correct data, it is risky to do this, because if the data ever changes in any of the cells, the answer will no longer be correct. For example, let's pretend that the Number columns corresponded to cars sold in February at an automotive dealership. Recently, they discovered a recording error, they did not sell just 2 and 3 cars, they sold 20 and 30! Updating the calculation table leads to the following results.
Row 2 has the correct sales numbers, but row 3 does not because the user used numbers instead of cell references. It is always wise in Excel to use cell locations instead of numbers, even if it means creating extra rows and columns. That way, if any of your data changes, Excel immediately can recalculate the information. This is also helpful for autofill.