Основная причина всё-таки - насторожить водителя. Потом идёт - оставить отпечатки лапы копа.
Reason 1: To Startle the Driver
Inevitably, police officers are going to run into unsafe situations when they pull people over. Sometimes drivers will have illegal substances or prohibited items in their cars like guns, ammunition, or drugs. Naturally, if the driver does have something illegal in the vehicle, he or she will want to hide it before the officer approaches the window.
So, one of the reasons why cops touch tail lights is to startle the person inside. The driver is likely not expecting the noise of the tap – which typically causes him or her to stop for a moment, giving the police officer additional time to witness what the driver is trying to hide (if there is anything).
The continuous practice of this surprise tactic has proven to increase the arrest of intoxicated drivers, sellers of prohibited substances, people found in possession of unlicensed firearms, and other road-related offenses.
However, the practice of touching the tail light of vehicles can put our police officers in a risky situation. This is because this practice exposes the police officer’s position making him or her susceptible to attack.