The composite materials the Subaru disc brake pads are constructed of are just softer than the smooth flat metal surfaces of the brake rotors they come in contact with. This contact is the friction that slows down the momentum of the vehicle. The disc brake pads were designed to be the braking component that absorbs the wear from the contact with the rotors on your Subaru. The ending result is that the disc brake pads slowly disintegrate and convert from the solid padded surface to brake dust. This brake dust should be handled with care and not inhaled or injected for any reason. As the disc brake pads wear over their expected lifetime, the piston in the brake caliper that forces them against the brake rotor has to move outward filling in space vacated by the material that has turned to dust. This repositioning of the piston required the hydraulic fluid to move from the reservoir in the brake master cylinder to the cylinder in the brake caliper. The greater the wear on the disc brake pads, the more fluid is required to be transferred on your Subaru. This is why the hydraulic fluid levels in the master cylinder the best indicator when your Subaru disc brake pads are nearing the end of their lifecycle. The lower the level is in the reservoir, the less amount of padding material in your brake system is present. With the Subaru owner not only being aware of this but checking this fluid level periodically, they will know about the time a brake job will be required on their vehicle so no damage can occur to their braking system. The only other way to know the amount of padding that is left in your braking system is to pull the wheels and look at them.