IMO they are mainly/only fitted and used by drifters,rally and stunt drivers (and wannabees )
Some parking brakes are not very effective when the operate on the rear discs that do not incorporate a small drum for the HB.
Hydraulic parking brakes as a sole means of operation are not acceptable on vehicles first used on or after 1 January 1968. However, they may be used to assist the application or release of a mechanical brake.
An electronic parking brake (EP, although applied electronically, must be maintained in operation by direct mechanical means. This mechanism may be within the brake caliper or within the motor/gear assembly, so cannot be readily seen.
Convenience and weight saving. The old system was a pull lever under the dash with a cable running to the back which pulled on a T bar which in turn pulled a 2 pairs of mini pads. Sytem only ever worked just before the annual MOT. The mods required to fit the Sierra Cosworth diff meant that this T bar had to be removed.
Designing and fitting a new system to operate the handbrake mechanism on the lightweight replacement calipers was gong to be a pain, so fitting a hydraulic brake was a no brainer. It also meant that I could remove the handbrake mechanisms from the calipers, further reducing weight
Nice !! Do you have to retain a rudimentary mechanical arrangement simply to comply with MoT requirements? Even though the hydraulic arrangement is far more effective
Legislation is odd. Insofar as your hydraulic arrangement will be superior to any cable mechanical arrangement yet the MoT requires an inferior system to be present.
I guess that the problem lies with the fact that the handbrake is also classed as an emergency brake independent of the hydraulic system, so if the hydraulic system fails you still have a mechanical backup. I have lost this but have a twin system with separate m/c cylinders for front and rear brakes so I still have a back up if one fails.
It would make sense for a completely separate hydraulic system to be allowed in place of a cable then. Cable operated systems are far more likely to fail than hydraulic ones in my experience. I have had far more handbrake bits replaced than I have ever had hydraulic bits.