If a driver is impaired by drugs or alcohol when he or she is operating a vehicle, it is against the law. This is because they are at a much higher risk of making an error of judgment, are subject to delayed reaction times and are more likely to engage in reckless or distracted driving. In order to enforce this law, drivers may be stopped and asked to take a Breathalyzer test, which can determine the level of alcohol in their blood.
A Breathalyzer test is not the most accurate way to test the blood alcohol concentration (BAC), but it is the most convenient, and it allows field sobriety tests to be conducted. They are usually conducted by a police officer, and they likely have a reason to believe that the driver is under the influence of alcohol; perhaps he or she was speeding or jumped a stop light.