Tail Light Tapping is a Surprise Attack
Inevitably, cops are going to face some drivers with illegal doings, and this is where tapping the tail light comes in handy. Police officers touch on the tail light as a surprise attack. If you’re a driver with prohibited items or illegal substance in your car, naturally, you’re going to hide everything from a cop.
Source: Washington Post
The sound from the tapping of the tail light startles the driver who is trying to hide something. If this happens, the cops will notice some weird behavior from the driver, which he could use to suspect the driver of illegal actions.