The power steering pump pulley is spun by the serpentine belt. It turns the pump's internal rotor to generate hydraulic pressure. The pulley is pressed onto the pump shaft with an interference fit, requiring a specialized tool to remove or install it.
Plastic pulleys crack from engine heat and belt tension. Metal pulleys can bend if hit during other engine work. A bent or cracked pulley will shred the serpentine belt, leaving you stranded without steering assist or alternator power.