As you probably know already, your brake pads do not last forever. Brake pads (and brake shoes on a drum brake system) contain sacrificial friction materials designed to wear away with use in order to protect other brake system components.
When your brake pads wear down, not only is it important to replace the pads, but also the hardware – clips, bushings, guides, et al. – that hold the pads in place and allow them to move freely. Sometimes even the brake rotors need to be replaced since they too wear out over time.