Copying the entire cell allows to copy to include other attributes so that, when pasted into another application that can understand them, you have the option of pasting in a different format. For example, in Word you can paste a cell as a picture of the cell, as a table cell or as an Excel object. Copying from the formula bar doesn't do that and gives you at most any font attributes (e.g. bold). Also, copying from the formula bar isn't a great deal of use if the cell contains a formula and you want to copy/paste the result, not the formula.