why are air bubble dangerous for hydraulic brake system?

In hydraulics, one depends on ability of hydraulic fluid to transfer energy from the input end of system to output end.
The energy associated with compression of the fluid at input end is ‘used up’ compressing a bubble in the line, thus that much energy won’t be going to output end.
Compression of fluid at input end results in compression of bubble rather than delivery of energy to output end.
But, in a braking system, this is a disaster because it leads to poorly performing brakes or even a failure, so on a moving vehicle, failure of brakes spells disaster.