Rustproofing will be helpful for many reasons: it protects a car body from the rust, it protects electrical connectors and wires from the moisture, it protects the brake lines from corrosion (which is very important for safety reasons). Look at the picture - this is located underneath part of the brake system, it's completely rusty as you can see. This is only five years old vehicle. One day such a rusty brake line will burst and the car will simply have no brakes.