These plates grind against one another, split, and collide producing many of the geological features we see on the Earth's surface today. Old crust is destroyed where plates collide as it subducts back into the Earth's interior. Mountain ranges are built where plates collide. It takes only 100 million years for erosion to erase a mountain range. If mountains were not continually built by plate collisions there would be no mountains on the Earth.
Earthquakes occur where plates move relative to one another.
In southern California two plates are grinding past one another. In about 20 million years Los Angeles will be an island suburb of San Francisco.