The steel powder coated plate is your printing surface. As it is steel, it is attracted to magnets or magnets are attracted to it. If you attached the magnetic portion to the steel plate with the adhesive, there would be nothing to hold the plate to the glass. By attaching the adhesive magnetic sheet to the printer, the steel plate is then available for printing and also easy removal and flexing to pop free your prints. Exercise care in attaching the adhesive to your printer, to ensure there are no bubbles or variations in the surface, as it will create an uneven surface on which to print.