Suppose, you need that Excel display actual today's day during each opening your file. So you can use embeded formula TODAY().
But exist more simple approach to do it. You can use hotkeys (Ctrl + ;).
But the difference is only in further displying. Function TODAY() every day changes data to actual value. If you have used hotkeys (Ctrl + ;) the date will display without any changes during further opening file (will display value of date at the date of entering).
Therefore use required approach depending on the your goals.