Found this basic explanation:
"Rubber Bushings
In stock form, almost all cars use rubber bushings in the suspension. The reason for this is that rubber bushings absorb much of the force produced by the vehicle's suspension when it is driven over bumps and road imperfections. This in turn isolates the occupants and provides a more comfortable, compliant ride. One of the issues with rubber bushings though is that, because they deflect more, the suspension can lose precision. This is generally not a problem for normal driving, but if a car is modified for better handling, a more rigid setup may be desired. Among the solutions to make a suspension more precise are urethane bushings as well as a more extreme solution, which are pillow ball mounts.
Pillow Ball Mounts
Despite their misleading title, pillow ball mounts are solid metal mounts that are used in areas such as the upper strut mount. The rubber normally used is replaced with a metal spherical bearing. Rather than deflecting from the cornering forces like a conventional rubber bushing, pillow ball mounts do not move at all. This translates into a far more precise feel from the suspension and steering.
Considerations
While pillow ball mounts definitely increase a vehicle's handling and steering response, they do so at the cost of comfort. The metal bearings will transmit far more vibrations from the road, which will make the vehicle less comfortable. They can also rattle and squeak. This may be fine if the car in question is not a daily driver and used for recreational driving or track driving. However, if the car is driven frequently on the street, pillow ball mounts may be too extreme."