Try it and you'll see that it hides whatever (almost) you type into that cell.
The number format of a cell consists of parts each separated by a semi-colon:
positive-number-format;negative-number-format;zero-format;text-format
Since there's nothing in those parts with ;;;
excel says to not show any positive, negative, 0, or text values.
One of the differences between a white-font on a white fill and a custom format
of ;;; is what happens when you autofit that column.
Try it with some long text in a few cells (A1:A5) and asdf in A6
Format a1:a5 with white on white and autofit
Format a1:a5 a second time as ;;; and autofit