This procedure is important because brake fluid is hydoscopic(absorbs moisture) and moisture is a huge enemy of the brake system. This moisture rusts wheel cylinders and calipers causing them to stick or leak. This is also a safety issue because water boils at a much lower boiling point than brake fluid, what this means is when the water in the system turns to a gas(steam) it becomes compressible and the brake pedal goes to the floor and you go for a ride! Brake fluid absorbs 2% moisture per year and should be flushed at the recommended intervals. The recommended service intervals are: yearly on cars equipped with anti-lock brakes, and bi-yearly on vehicles without antilock brakes.