Vehicles used to give some sort of feedback or warning signals to the driver when it was time for a tune up or general service but with new cars, that's no longer the case. So if owners have a "if it ain't broke, then why fix it" mentality, it can mean those regular maintenance jobs get missed - like changing brake fluid.
Why bother if the brake pedal feels fine and there were no braking issues in the past when it came to Warrant of Fitness time?
Hydraulic brakes work on the theory that you can't compress a liquid, so a specific type of brake fluid is used which has a high boiling point to withstand the temperatures that can at times build up in the brake system.
For example, heavy and repeated braking when descending for long periods can raise the brake fluid temperatures significantly. The danger of not changing the fluid regularly is its boiling is lowered because brake fluid is hygroscopic (meaning it has the ability to attract and hold water molecules from the surrounding environment) and therefore can, and will over time, absorb moisture.
So when the brakes get a hard workout and the fluid temperatures rise, water within the system can reach its boiling point and vaporise. While you can't compress a liquid you can compress a vapour - the end result is a spongy brake and a drop in braking performance.
While brake fluid has corrosion inhibitors added, over time they will become less effective which leaves the internals of a brake system vulnerable to corrosion because of the introduction of the moisture.
Once the brake fluid has been changed are you going to notice any immediate difference? The answer is highly unlikely, but if changed for the right reasons it will make the vehicle a lot safer and reduce the risk of brake-related problems caused by corrosion further down the track.
Note: Take your car to your mechanic to change the brake fluid, as it is not a recommended DIY job on the modern motor vehicle.