Perhaps you left a local bar after having beers with colleagues. You roll through a stop sign, and an officer pulls you over. The officer notes your slurred speech and inability to make eye contact, so they ask you to perform a field sobriety test. You believe you pass, yet the officer asks to test your BAC level with a breathalyzer.
You may not want to refuse. Perhaps you believe that by the time an officer arrests you and brings you to the station for further testing, your BAC level may test low enough to pass. Unfortunately, because of the law of implied consent, you may already be charged with a crime in addition to a DUI.