They may be a left over relic from overheating drum brakes in HGVs and the like?
This.
If any vehicle ends up in one now, the vehicle will likely be getting inspected to find out why it ended up in the gravel.
Modern* brakes should not fail under normal use, unlike the brakes of 30+years ago that were often incapable of stopping fully loaded vehicles on anything resembling a descent.
*unless you happen to be in the US, where they are still using that same technology from 30 years ago, as brake discs and electronic braking are works of the devil, and to be avoided at all cost as they’re too complicated/not as good/more expensive/<insert other (generally invalid) excuse to justify avoidance of safety improvements>