Power-assisted steering makes manoeuvring in cities and confined spaces much easier. It also lets you move the steering wheel with much less effort when your car is moving slowly or even stationary. Power-assisted steering is only an additional feature, so even if the system fails, the steering wheel and the wheels are still linked and can be moved, it just becomes heavier.
When your car has power-assisted steering, its engine powers a pump which sends oil to the rack or the steering box. Valves in the rack or box open whenever you turn the steering wheel allowing oil into the cylinder. The oil pushes the steering in the appropriate direction. When you stop turning the wheel, the valves close again.