Without getting into technical jargon,
the digitizer is simply the piece that is fused to the glass screen that recognizes the user`s human touches and translates these physical requests into computable actions.
If you damage the front panel screen,
including the digitizer such that your iPhone or iPod isn`t properly or accurately recognizing your touches on the touchscreen,
then it is in your best interest to do a complete replacement of the LCD,
the digitizer and glass itself,
as doing a full repair will ensure that they all work together.