Less safety for the speakers, more protection for the output stage.
Those breakers trip with excess current- not voltage, so transients can pass without premature tripping. Excess current can come from too many paralleled speakers, low impedance speakers turned up too high for too long, or simply a shorted speaker lead.
The 'usual relay' you mention can be various things. It can be simply a delay at turn on, an overpower/overcurrent arrangement or detecting and protecting against DC (output stage failure) and, most often, a combination of all three.
HKs implementation using circuit breakers is valid, reliable, and simple. In the case of the A-402, there is no DC protection implemented, however there is turn-on delay thump protection implemented by a simple AC detect/delay which turns on the differential pair (input stage) after a few seconds.
HK's attitude was always to protect their amps, not your speakers.