Ever so often you may wish to hide a cell's content from the user's inquisitive eyes.
One way to achieve that result is to set font and background colors to the same value. Another is to format the cell as ";;;". (Format/Cells/Number -> Custom -> Enter ;;; in the Type field).
The difference between the two is in that the cell retains its format in the first case but not in the other. If you have your cell's column width set to expand to a size required by a cell's content the invisible cell can trigger such an adjustment. Not so in the second case because the format ;;; reduces the display to zero length.