Why brake pads need replacing
Safety-critical is one way to describe your car's brake pads, and that's why they should be maintained to avoid a potential disaster. If the pads wear down to the backing plate, your car won't stop as quickly as you want it to, which could ruin both your day and that of anyone ahead of you.
The friction material on the pads does, in time, wear down. You'll need to replace them before the steel backing plates rub the rotors, 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 that hits the rotor 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.