Over time brake fluid wears out, due to the nature of the brake system and fluid it self. The brake system on a car produces heat when stopping; both from the friction of the brake shoes or pads and from the hydraulic force used to apply them.
Brake fluid, used in most cars, is hygroscopic. This means it draws moisture from the air. This extra moisture can corrode the metal components in your brake system and reduce their lifespan.
Old brake fluid, reduces your braking performance. This may not be noticeable in typically driving, but emergency situation, every inch counts to avoid a collision.
ABS components are sensitive to moisture with the braking system. Changing your fluid reduces the risks of damage to these components.