Your car is tested to insure that it's continuing to run as clean as it was designed to, and if it isn't, to identify which pollutants are being produced in excessive amounts, and need to be brought under control by having your engine serviced. Your car's emissions does make a difference, and it's a difference that will ultimately benefit you.
Modern cars control emissions two ways.
First, they're designed to burn gas more efficiently than ever before. This automatically reduces the amount of pollution the engine produces, since pollution is typically caused by inefficient combustion. Second, today's cars have special devices within them that are designed to reduce emissions output even further. When your engine is running, sensors in and around it continually detect and analyze an amazing range of information - from engine load to the amount of oxygen in your exhaust, and much more. The information these sensors transmit to your on-board computer tells the computer how to set and control performance components that include your car's fuel injectors or fuel pump relay, idle air and speed controls, catalytic converter and dozens more.