In days past you would never hear of such things as replacing your brake fluid. Now, with the introduction of anti lock brakes it has to be done. Failure to do so can cause faults within the ABS, Traction, and Stability Control systems of your vehicle.
Brake fluid is hydroscopic. That means that it absorbs moisture. In addition to absorbing moisture it also runs through copper lines and inevitably transfers residual or microscopically fragmented copper through the brake system.