The cable operated parking (or sometimes called "emergency") brake is a complete separate brake system acting only on the rear wheels. When the lever in the car is pulled up, cables running to the rear of the car actuate 2 sets of brake shoes. These shoes expand against the machined surface inside the rear brake disc. Thus, the car still has (limited) stopping ability even if the entire hydraulic brake system fails.
Because of the many advances in design and engineering, and the government regulations requiring dual hydraulic brake systems, the odds of a total hydraulic failure are probably longer than your chances of hitting the lottery in three states on your birthday. The system must, however, remain in proper repair and adjustment so that it will hold the car when parked and be available for emergency use if needed.