When you turn your trucks, most standard cast trucks will deform the bushing. The bushing will move around against the kingpin, they will bulge or even worse split. This effects your ride, because the bushing will not perform the same way each turn, create some unpredictability and a choppy performance. This washer solves a lot of those problems.
It is machined to an exact size, and it has a sleeve that fits inside the bushing. The result is the bushing is pre-loaded, which is fancy talk for it already has some tension like you are turning. Immediately the bushing will hold its shape and perform the way it was designed. You'll notice the truck has a more nuanced turn, the removal of that jerkiness, better lean through the truck. The bushing also has more responsiveness and bounce. Come out of that tight corner and it will centre quicker, almost bounce out of the turn.
With bushings and precision washers it is always best to experiment with what feels best on your set up. These are a few generic suggestions.
Freeriding: flat washers all round = boardside, roadside - front and back
Fast freeride: Cupped washer on back boardside and flat washers on the rest
Downhill symetrical: Flat washer roadside and cupped washer boardside front and back
Downhill directional: Back truck two cupped washers, front truck two flat washers