The original bushing is made of rubber: it filters vibrations well, but it deforms under load. When cornering or braking, the arm moves by a few millimetres and the alignment changes during the manoeuvre - this is the slop between the steering wheel and the wheels.

Polyurethane deforms much less: the connection becomes precise and repeatable, and the car responds at the same point every time. In return, you will get more road noise and vibrations in the cabin, especially when cold.