It is illegal in California as well to use flashers while moving. Doubt most people actually take time to review the DMV guide and once they have their driver's license renewal is for the most part automatic.
Danger is that motorists will come to think that when approaching a vehicle that it is simply moving slower than other traffic when in fact it may be at a standstill. Most highway patrol casualties are from motorists that are not paying attention and drive into the back of a patrol car that is parked beside the road.
Second officers are often used as spotters and they will stand far away from the patrol car and the car/motorist that has been pulled over so that they are out of harm's way and can shout out to the officer if another car appears to be headedd their way. It is also why the officers in California at least now stand on the passenger side of the car.