When police officers pull a motorist over for a basic traffic stop, it's customary for them to reach out and touch the taillight of the vehicle before approaching the driver's window. Why? There are two answers, and they're both pretty interesting.
According to TheLawDictionary.org, the most common reason is so an officer can put their fingerprints on the car. If the driver attacks the officer and/or flees, the officer will have physical evidence to prove which car he pulled over. Dashboard cameras have made this practice less essential, but officers still like to do it.
A second reason is because officers used to also give the taillights a slap to distract the driver if they're frantically trying to hide drugs, illegal weapons, or other unlawful items before the officer gets to their windows. More recently, this practice has been discouraged because it instantly clues a potentially uncooperative driver into the officer's position.
The reality of both these reasons is that the men and women who serve and protect us have to also protect themselves in a society that is becoming increasingly hostile toward them.