hi, welcome.
as this was a 'learner' bike, they figured a parking brake to stop it creeping at lights (really) or leaving it parked i guess on a steep hill with no kerb to rest the rear wheel on
Ive covered 5000 miles on my cb50a, and never needed it once.
if you've ridden other bikes you'll be fine.
(copied this..To use the parking brake, apply the rear pedal while pulling out on the knob on the left side. To release, depress the button and push in the knob and push down on the pedal to release the brake.)