Brake fluid is hygroscopic,
It is designed to operate in a semi closed system under pressure.
the reason to replace the fluid is to ensure the water content it naturally absorbs does not impair its function when heated (i.e. under heavy breaking )
Similarly increase water content can cause corrosion to the internal brake components such as slave cylinders , pistons , brake lines etc,
There is no requirement to replace brake fluid at pre determined intervals, other than the extra revenue achieved by the dealer.
Brake fluid can be tested for water content very simply and within a minute.
Usually checking is carried out annually, two years after the last fluid change. Usually modern brake fluid is good for more than two years (lots more)
when the fluid is found to have elevated water content, approaching the predetermined levels & via the metered test, its is then changed. (Not before )
VW dealers take the opportunity to upsell this change at specific intervals "Recommend " to change for safety reasons ! (Scare tactic)
This is highly lucrative for the Main dealer as they charge a ridiculous amount compared to other highly competent garages.
Also binning perfectly good brake fluid is not environmentally friendly and purely cost the consumer unnecessarily !
I'm sure some will come on here to defend the practice , but in reality a simple one minute test can determine if the brake fluid needs to be changed.
Why change before it needs to be changed , the degradation is a very slow process, which can be monitored upto a level which is Considered normal brake fluid operating parameters.
Similarly Not one single brake components are not covered under warranty, therefore "recommended maintenance" is purely a safety consideration & one that should not be an issue for a vehicle within the first three years (See simple annual test) if it were recognised as an issue within the first three years, surely they would make it a mandatory change within the service schedule, not a recommendation!
It's your choice if you want to be ripped off by the main dealer pricing or have the fluid checked properly by a trusted garage and help save the environment and your pocket.