A parking brake system is not required if your service brakes are fully functional?

In the UK the parking brake is called the handbrake, a motor vehicle must, by law, have at least two independent ways of applying the brakes. This is normally achieved on a car by having the foot brake hydraulically or, rarely, pneumatically operated and the hand brake cable operated.

Added: (in the US) you must also have an operational parking (or hand) brake.
In Canada, at least, (and I strongly suspect in USA and Australia) that is wrong. You DO require a parking brake as an emergency standby. The point is your regular brakes depend on hydraulic fluid and rubber seals, while the parking or emergency brake depends on cables. -A totally separate system.

You also need the parking brake when parking the vehicle on an incline. The transmission park pawl may break causing the vehicle to roll off if you have no parking brake.