It's not surprising that Brake Fluid tends to be one of the most neglected components of the braking system. Brake fluid isn't very visible. But when you consider the job it does, it does become clear why it needs changing and should be upgraded.
Brake Fluid is "hygroscopic". It absorbs water over time. Because the boiling point of brake fluid tends to be much higher than that of water, you can find your brake pedal becoming spongey as the water in the system boils into steam. You don't want this to happen and most manufacturers recommend a fairly short service interval of just a couple of years for brake fluid to prevent it.
There's also different grades of brake fluid according to how hard you're expecting to work the system. Our specialist track brake fluids can have dry boiling points of over 300 degrees. If you're upgrading other parts of your braking system, you should make sure your brake fluid is not the weak link. If you're running standard parts, you should change it to schedule to make sure it's working properly. The DOT grade of brake fluid tells you how hot it can get.