Why brake pads need replacing
Brake pads are critical for the safe operation of your car, and should be maintained to avoid a potential disaster. If the pads wear down to the backing plate, stopping sistances will be a lot further and may cause an accident.
The friction material on the pads is slightly softer than the steel discs and with time, the pads wear down. Starting with more than 1/4 inch of material, a microscopic amount is rubbed off every time you stop. You'll need to replace them before the backing plate are rubbing the discs, or you will quickly need to replace the discs as well.
Some cars have brake pad wear indicators, which activate a light on the dashboard when the pads need replacing. A simpler wear indicator is a little metal tab which hits the disc and starts to squeal when they are 75% worn out. Another way of telling how worn the pads are is to examine the level of fluid in the brake fluid reservoir (which drops as the pad wears).
Haynes recommends inspecting the pads and measuring the wear whenever you have the wheels off to rotate the tires. This task requires some experience, but uses basic tools, and will take just a minute per wheel, if you already have the wheels off.