Why do police officers stop for violations like tag lights, & turn signals?
As stated before, most officers do what the City, State, and Federal government require them to do. Tag lights and turn signals might not seem like a big deal, but they are covered under state law. They also give an officer a chance to see if a driver is under the influence of drugs or alcohol, and to check for warrants.
Just remember Oklahoma Trooper Charlie Hanger stopped a vehicle on April 19, 1995 for a simple tag violation. He ended up arresting the driver, who was later identified after a warrant was issued as Timothy McVeigh (Alfred P. Murrah Building/The Oklahoma Bomber).