Taillight popping was also down in the hopes of catching the driver by surprise as it was a prehistoric age back then before dash cams were around! The little tap of the taillight was done in hopes of startling the suspect just long enough for the officer to see what suspect might be hiding.
Think about it, if a suspect is already nervous, hearing a tap of the taillight might just be the crack needed for the suspect to lose nerve and spill the beans! Far fetched we know but hey some cops swear by it’s effectiveness in shutting down suspects.
There IS a second reason other than the hopes of stalling a suspect hiding something. The other side of it is to leave evidence that the cop was there in the horrible chance that the suspect would attempt to incapacitate the officer.
Leaving his fingerprints behind is a way for the officer to leave evidence so that if there was a fatal accident the fingerprint could be matched in case solving to prove the murderer. That’s some Sherlock Holmes for ya right there!