What is the primary function of trailer lights on combination vehicles?

The primary function of trailer lights on combination vehicles is to signal turns and stops. This is crucial for safety because these lights inform other drivers of the vehicle's actions on the road. When a driver intends to make a turn or come to a stop, the trailer lights provide a clear indication of those intentions to vehicles behind and alongside. This reduces the risk of accidents by making the movement of the vehicle more predictable and visible to others.

While the other options might seem relevant in certain contexts, they do not encapsulate the main safety purpose of trailer lights. For instance, while aesthetics can play a role in a vehicle's overall appearance, it is not the primary reason for the existence of trailer lights. Similarly, although increasing visibility in daylight can be a benefit of trailer lights, their core function relates directly to communication on the road rather than merely enhancing visibility. Reflective markers serve a different purpose, as they are designed to reflect light and make the vehicle visible in low-light conditions, but they do not actively signal the vehicle's movements.