[SteveC]

Over time and regardless of mileage, brake fluid absorbs water from the atmosphere. Under severe braking, the brake fluid temperature rises above 100 degrees and the water boils and as we all know, steam is compressable, so your brakes become less effective.
In 99% of cases, this won't be a problem, as you'll rarely encounter such extreme braking situations. On the other hand, an Alpine pass is not the time to discover that you should have changed your brake fluid