Excel worksheets can be full of formulas and functions and in many instances you did not create them and are trying to decipher how a formula was created. It might also be that you created it some time ago and you are reviewing what you did.
You could simply click on the cell and look at the formula or function and see what cells are being used. Alternatively you might have a value and you want to check on where it is being used.
The other thing you might just want to check if you set up a formula correctly or showing someone how you did it.
This is where formula auditing comes in, it is really easy to use and it visually shows you what cells are being used in the formula or you can click on a cell and see where that value is being used as Excel will draw arrows on the worksheet for you to see what is going on.