Trailer lights are extremely important I can name several times I was in a vehicle and the trailer in front of us didn't have operable taillights.
The first, and only I'll share here.
Sunny, fair weather, a few scattered clouds, and the day before it had rained for several hours, but the road was mostly dry.
Pick-up with a fifth wheel camper going Southbound being followed by us, also in a pick-up towing a fifth wheel toy hauler. A small sedan passed another pick-up/5th wheel in a no passing zone on an undivided highway going Northbound. The pick-up/5th wheel in front of us slammed on his brakes to avoid hitting the sedan, who otherwise would have ended up crushed between the fifth wheel and a semi who was in front of the fifth wheel. It took a fraction second to see that the truck and trailer was stopping in front of us since his taillights on the trailer were out. After slamming on our brakes and having to use the Southbound lane to keep from jackknifing. (that lasted about a second after we realized the truck coming the other way was using his whole lane.) We ended up stopping about a dozen yards behind the other trailer. We had plenty of room left, but it wasn't a fun experience.
Everyone was driving exactly as they should have been except the sedan that passed. Following distance for conditions, speed, and all other considerations were accounted for by all drivers except the sedan. It was that fraction of a second realizing the guy in front of us was stopping (and doing it quick) that caused us to have to come to such a violent stop. Had he made sure his lights were on before leaving we would have been able to stop sooner and without using the other lane. For those wondering why the trailer wanted to slide it was because, at the time, the brakes locked-up very easily. No matter how much we tested with the brake controller they locked up, it finally took a few thousand miles for the brakes to stop doing that.
Attached is a rough sketch of the scene. Top is NB and the bottom us SB. Not to scale.