One aspect of Excel’s protection is the ability to hide the contents of a cell. The cell still displays, but the formula bar remains empty when the cell is selected. This is useful if you don’t want the users to see the formula for the cell.
On the Protection tab of Format>Cells, check the Hidden box.
Then protect the sheet using Tools>Protection>Protect Sheet. With the Hidden property checked and the sheet protected, the formula is no longer visible to the user.