Whatsthisbuttondo.
Its a regulation requirement to have an independent backup brakes source.
On a hydraulic dependent a/c the RAT will not supply enough energy to supply the brakes. Whether it has a hydraulic pump or a generator driving a pump etc.
The 787 requires an independent backup power source for its braking system. The battery is it.
It matters not a jot whether you or I calculate the odds of it being needed as 10 milllion to 1 or evens.
The regs say it needs to be there. End of.