[blattimwind]

Classic handbrakes use a braking wire and don't use the hydraulic breaking system (so it's somewhat redundant, actually). Also, the brake fluid is inside the system and can't prevent the brake cylinder or the brake pads from seizing.

[AstralStorm]

The difference is the handbrake will seize off rather than braking, which means the car is usable or at least easier to haul.